I’ve noticed over the years that while VMware admins tend to really understand maintenance mode, a lot of others in adjacent spaces (storage, network, etc.) have a very murky perspective on it. In fact, I’d bet that if you sat down a storage person at vCenter and told them to evacuate a host with MM, odds are they would be really confused when the task hung or failed. I know I was the first time I tried it. Continue reading
Or, more accurate than the title, a black area on the border of the console window.
I got a new laptop running Windows 10 and when I brought up the console in my home lab, this is what I’d get.
It actually ended up being worse even than it looks as the mouse is pretty much unusable on the screen and a huge chunk of the screen (bottom and right side) is cut off. It is virtually impossible to do anything with VMs including installations of operating systems.
I tried several fixes like compatibility mode, update drivers, etc. without success. Then I stumbled upon this scaling setting which is mentioned on several sites dealing with mouse usage in the VDI space.
Browse to C:\Program Files (x86)\VMware\Infrastructure\Virtual Infrastructure Client\Launcher\ and right click on VpxClient.exe and select Properties.
On this screen, select the Disable Display Scaling box indicated. Then restart your client.
As a annoying side effect, now some of the text isn’t scaled right so some things in the client look wrong. But it is still usable and the console is usable.
Weirdly, the vCenter console from the web client has no such issue that I noticed. But if you are being pestered by this “feature” try this setting and see if it helps.
I’ve been noticing some fundamental misunderstandings around storage encryption – I see this most when dealing with XtremIO although plenty of platforms support it (VNX2 and VMAX). I hope this blog post will help someone who is missing the bigger picture and maybe make a better decision based on tradeoffs. This is not going to be a heavily technical post, but is intended to shed some light on the topic from a strategic angle.
Hopefully you already know, but encryption at a high level is a way to make data unreadable gibberish except by an entity that is authorized to read it. The types of storage encryption I’m going to talk about are Data At Rest Encryption (often abbreviated DARE or D@RE), in-flight encryption, and host-based encryption. I’m talking in this post mainly about SAN (block) storage, but these concepts also apply to NAS (file) storage. In fact, in-flight encryption is probably way more useful on a NAS array given the inherent security of FC fabrics. But then, iSCSI, and it gets cloudier.
Before I start, security is a tool and can be used wisely or poorly with equivalent results. Encryption is security. All security, and all encryption, is not great. Consider the idea of cryptographic erasure, by which data is “deleted” merely because it is encrypted and nobody has the key. Ransomware thrives on this. You are looking at a server with all your files on it, but without the key they may as well be deleted. Choosing a security feature for no good business reason other than “security is great” is probably a mistake that is going to cause you headaches.
Here is a diagram with 3 zones of encryption. Notice that host-based encryption overlaps the other two – that is not a mistake as we will see shortly.
Data At Rest Encryption
D@RE of late is typically referring to a storage arrays ability to encrypt data at the point of entry (write) and decrypt on exit (read). Sometimes this is done with ASICs on an array or I/O module, but it is often done with Self Encrypting Drives (SEDs). However the abstract concept of D@RE is simply that data is encrypted “at rest,” or while it is sitting on disk, on the storage array.
This might seem like a dumb question, but it is a CRUCIAL one that I’ve seen either not asked or answered incorrectly time and time again: what is the purpose of D@RE? The point of D@RE is to prevent physical hardware theft from compromising data security. So, if I nefariously steal a drive out of your array, or a shelf of drives out of your array, and come up with some way to attach them to another system and read them, I will get nothing but gibberish.
Now, keep in mind that this problem is typically far more of an issue on a small server system than it is a storage array. A small server might just have a handful of drives associated with it, while a storage array might have hundreds, or thousands. And those drives are going to be in some form of RAID protection which leverages striping. So even without D@RE the odds of a single disk holding meaningful data is small, though admittedly it is still there.
More to the point, D@RE does not prevent anyone from accessing data on the array itself. I’ve heard allusions to this idea that “don’t worry about hackers, we’ve got D@RE” which couldn’t be more wrong, unless you think hackers are walking out of your data center with physical hardware. If the hackers are intercepting wire transmissions, or they have broken into servers with SAN access, they have access to your data. And if your array is doing the encryption and someone manages to steal the entire array (controllers and all) they will also have access to your data.
D@RE at the array level is also one of the easiest to deal with from a management perspective because usually you just let the array handle everything including the encryption keys. This is mostly just a turn it on and let it run solution. You don’t notice it and generally don’t see any fall out like performance degradation from it.
In-flight encryption is referring to data being encrypted over the wire. So your host issues a write to a SAN LUN, and that traverses your SAN network and lands on your storage array. If data is encrypted “in-flight,” then it is encrypted throughout (at least) the switching.
Usually this is accomplished with FC fabric switches that are capable of encryption. So the switch that sees a transmission on an F port will encrypt it, and then transmit it encrypted along all E ports (ISLs) and then decrypt it when it leaves another F port. So the data is encrypted in-flight, but not at rest on the array. Generally we are still talking about ASICs here so performance is not impacted.
Again let’s ask, what is the purpose of in-flight encryption? In-flight encryption is intended to prevent someone who is sniffing network traffic (meaning they are somehow intercepting the data transmissions, or a copy of the data transmissions, over the network) from being able to decipher data.
For local FC networks this is (in my opinion) not often needed. FC networks tend to be very secure overall and not really vulnerable to sniffing. However, for IP based or WAN based communication, or even stretched fabrics, it might be sensible to look into something like this.
Also keep in mind that because data is decrypted before being written to the array, it does not provide the physical security that D@RE does, nor does it prevent anyone from accessing data in general. You also sometimes have the option of not decrypting when writing to the array. So essentially the data is encrypted when leaving the host, and written encrypted on the array itself. It is only decrypted when the host issues a read for it and it exits the F port that host is attached to. This results in you having D@RE as well with those same benefits. A real kicker here becomes key management, because in-flight encryption can be removed at any time without issue. You can remove or disable in-flight encryption and not see any change in data because at the ends it is unencrypted. However, if the data is written encrypted on the array, then you MUST have those keys to read that data. If you had some kind of disaster that compromised your switches and keys, you would have a big array full of cryptographically erased data.
Host Based Encryption
Finally, host-based encryption is any software or feature that encrypts LUNs or files on the server itself. So data that is going to be written to files (whether SAN based or local files) is encrypted in memory before the write actually takes place.
Host-based encryption ends up giving you both in-flight encryption and D@RE as well. So when we ask the question, what is the purpose of host-based encryption?, we get the benefits we saw from in-flight and D@RE, as well as another one. That is the idea that even with the same hardware setup, no other host can read your data. So if I were to forklift your array, fabric switches, and get an identical server (hardware, OS, software) and hook it up, I wouldn’t be able to read your data. Depending on the setup, if a hacker compromises the server itself in your data center, they may not be able to read the data either.
So why even bother with the other kinds of encryption? Well for one, generally host-based encryption does incur a performance hit because it isn’t using ASICs. Some systems might be able to handle this but many won’t be able to. Unlike D@RE or in-flight, there will be a measurable degradation when using this method. Another reason is that key management again becomes huge here. Poor key management and a server having a hardware failure can lead to that data being unreadable by anyone. And generally your backups will be useless in this situation as well because you have backups of encrypted data that you can’t read without the original keys.
And frankly, usually D@RE is good enough. If you have a security issue where host-based encryption is going to be a benefit, usually someone already has the keys to the kingdom in your environment.
Hopefully that cleared up the types of encryption and where they operate.
Another question I see is “can I use one or more at the same time?” The answer is yes, with caveats. There is nothing that prevents you from using even all 3 at the same time, even though it wouldn’t really make any sense. Generally you want to avoid overlapping because you are encrypting data that is already encrypted which is a waste of resources. So a sensible pairing might be D@RE on the array and in-flight on your switching.
A final HUGELY important note – and what really prompted me to write this post – is to make sure you fully understand the effect of encryption on all of your systems. I have seen this come up in a discussion about XtremIO using D@RE paired with host-based encryption. The question was “will it work?” but the question should have been “should we do this?” Will it work? Sure, there is nothing problematic about host-based encryption and XtremIO D@RE interacting, other than the XtremIO system encrypting already encrypted data. What is problematic, though, is the fact that encrypted data does not compress, and most encrypted data won’t dedupe either…or at least not anywhere close to the level of unencrypted data. And XtremIO generally relies on its fantastic inline compression and dedupe features to fit a lot of data on a small footprint. XtremIO’s D@RE happens behind the compression and deduplication, so there is no issue. However host-based encryption will happen ahead of the dedupe/compression and will absolutely destroy your savings. So if you wanted to use the system like this, I would ask, how was it sized? Was it sized with assumptions about good compression and dedupe ratios? Or was it sized assuming no space savings? And, does the extra money you will be spending for the host-based encryption product and the extra money you will be spending on the additional required storage justify the business problem you were trying to solve? Or was there even a business problem at all? A better fit would probably be something like a tiered VNX2 and FAST cache which could easily handle a lot of raw capacity and use the flash where it helps the most.
Again, security is a tool, so choose the tools you need, use them judiciously, and make sure you fully understand their impact (end-to-end) in your environment.
That’s me blowing the cobwebs off of my blog. 🙂
Time marches on and things are always changing. I too have recently made several changes both in my personal and professional life. I am now a delivery engineer for CDI Southeast, though we are going through another transition period ourselves. I am also doing a bit of retooling, attempting to become a little less storage focused and branching out into other areas. Right now I’m trying to focus more on vSphere but also working with some DevOps stuff as well. I also released a course on Pluralsight last October which I am very proud of. If you are interested in hearing me talk to you about VNX for hours, I encourage you to check it out.
So, great things hopefully coming down the pipe for me in the future and I hope to continue to share with the community at large that has given so much to me.
In the meantime, here is a quick nugget that I turned up a couple of weeks ago. I was doing a pretty straightforward implementation of vSphere replication (and I hope to do a series on that soon), but ran into an oddity which I initially wrote off as a squirrel.
A squirrel is a situation where I walk into your building to do an implementation, and shortly thereafter you (or your co-worker, or your boss) comes up to me and says, “you know ever since you got here, the squirrels have been going crazy outside. What did you do?”
The squirrel can be anything really, but most of the time is comically unrelated to anything I’m actually working on. And sometimes it is just people messing around with me. “Hey you know Exchange is down? JUST KIDDING!”
Side note – I’ve been on site with a jokey client where something really did go down and it took us a minute or two to figure out that there was really a problem.
I’m used to squirrels now. Honestly they rarely have merit but I always take them seriously “just in case.”
In this case, in the midst of replicating VMs in the environment, an admin asked me if anything I was doing would mess with the clock on a guest server. I racked my brain for a moment and replied that I wouldn’t think so. The admin didn’t think so either so he went off to investigate more. I did some more thinking, and then went back and got some more information. What exactly was happening to the clock?
In this case the server had a purposefully mis-set clock. As in, it wasn’t supposed to read current time but it kept getting set to the current time. VMware tools dawned on me, because there is a clock sync built into tools that has to be disabled. We double checked but it was disabled. It made sense because we didn’t do anything with tools (no new or updated tools install).
So later that night I was playing around in my lab. I recreated the setup as best I could. Installed a guest with tools, disabled time sync, and set the clock back a year and some months. Then I started replication. And instantly, the clock was set forward.
So it turns out that even if you tell the guest “don’t sync the clock to the host,” it will STILL sync the clock to the host in certain situations.
While I understand the rationale (certain operations have the potential to skew the clock, ergo syncing the clock up after those will help prevent ongoing skew) I really feel like if time sync is disabled, it shouldn’t sync the clock. Ever. Or there should be another check box that says “No really, never sync the clock.” Nevertheless, I don’t work for VMware so I can’t tell them how to run their product.
In this case the fix is pretty simple though it does require downtime. Shutdown the guest and add some lines to the .vmx:
time.synchronize.continue = "0" time.synchronize.restore = "0" time.synchronize.resume.disk = "0" time.synchronize.shrink = "0" time.synchronize.tools.startup = "0" time.synchronize.tools.enable = "0" time.synchronize.resume.host = "0"
Now it will really, really never mess with the clock on the guest.
This might be common knowledge to VMware admins but I had no idea and I suppose I’ve never dealt with a purposefully skewed clock before.
In this final post we are going to cover a simple recovery, as well as do a quick summary. I’ll throw in a few bonus details for free.
Our CG has been running now for over 48 hours with our configuration – 48 hours Required Protection Window, 48 max snaps, one snap per hour. Notice below that I have exactly (or just under, depending on how you measure) a 48 hour protection window. I have one snap per hour for 48 hours and that is what is retained. This is because of how I constructed my settings!
If I reduce my Required Protection Window to 24 hours, notice that IMMEDIATELY the snaps past 24 hours are nuked:
The distribution of snaps in this case wouldn’t be different because of how the CG is constructed (one snap per hour, 48 max snaps, 24 hour protection window = 1 snap per hour for 24 hours), but again notice that the Required Protection Window is much more than just an alerting setting in RP+XtremIO.
Alright, back to our recovery example. Someone dumb like myself ignored all the “Important” naming and decided to delete that VM.
Even worse, they decided to just delete the entire datastore afterwards.
But lucky for us we have RP protection enabled. I’m going to head to RP and use the Test a Copy and Recover Production button.
I’ll choose my replica volume:
Then I decide I don’t want to use the latest image because I’m worried that the deletion actually exists in that snapshot. I choose one hour prior to the latest snap. Quick note: see that virtual access is not even available now? That’s because with snap based promotion there is no need for it. Snaps are instantly promoted to the actual replica LUN, so physical access is always available and always immediate no matter how old the image.
After I hit next, it spins up the Test a Copy screen. Now normally I might want to map this LUN to a host and actually check it to make sure that this is a valid copy. In this case because, say, I’ve tracked the bad user’s steps through vCenter logging, I know exactly when I need to recover. An important note though, as you’ll see in a second all snapshots taken AFTER your recovery image will be deleted! But again, because I’m a real maverick I just tell it go to ahead and do the production recovery.
It gives me a warning that prod is going to be overwritten, and that data transfer will be paused. It doesn’t warn you about the snapshot deletion but this has historically been RP behavior.
On the host side I do a rescan, and there’s my datastore. It is unmounted at the moment so I’ll choose to mount it.
Next, because I deleted that VM I need to browse the datastore and import the VMX file back into vCenter.
And just like that I’ve recovered my VM. Easy as pie!
Now, notice that I recovered using the 2:25 snap, and below this is now my snapshot list. The 3:25 and the 2:25 snap that I used are both deleted. This is actually kind of interesting because an awesome feature of XtremIO is that all snaps (even snaps of snaps) are independent entities; intermediate snaps can be deleted with no consequence. So in this case I don’t necessarily think this deletion of all subsequent snaps is a requirement, however it certainly makes logical sense that they should be deleted to avoid confusion. I don’t want a snapshot of bad data hanging around in my environment.
In summary, it looks like this snap recovery is fantastic as long as you take the time to understand the behavior. Like most things, planning is essential to ensure you get a good balance of your required protection and capacity savings. I hope for some more detailed breakdowns from EMC on the behavior of the snapshot pruning policies, and the full impact that settings like Required Protection Window have in the environment.
Also, don’t underestimate the 8,192 max snaps+vols for a single XMS system, especially if you are managing multiple clusters per XMS! If I had to guess I would guess that this value will be bumped up in a future release considering these new factors, but in the meantime make sure you don’t overrun your environment. Remember, you can still use a single XMS per cluster in order to sort of artificially inflate your snap ceiling.
A couple of things of note.
First, in my last post I stated that I had notice a bug with settings not “sticking.” After talking with a customer, he indicated this doesn’t have to do with the settings (the values) but with the process itself. Something about the order is important here. And now I believe this to be true because if I recreate a CG with those same busted settings, it works every time! I can’t get it to break. 🙂 I still believe this to be a bug so just double check your CG settings after creating.
Second, keep in mind that today XtremIO dashboard settings display your provisioned capacity based on volumes and snapshots on the system, with no regard for who created those snaps. So you can imagine with a snap based recovery tool, things get out of hand quickly. I’m talking about 1.4PB (no typo – PETAbytes) “provisioned” on a 20TB brick!
While this is definitely a testament to the power (or insanity?) of thin provisioning, I’m trying to put in a feature request to get this fixed in the future because it really messes with the dashboard relevance. But for the moment just note that for anything you protect with RP:
- On the Production side, you will see a 2x factor of provisioning. So if you protected 30TB of LUNs, your provisioned space (from those LUNs) will be 60TB.
- On the Replica side, you will see a hilarious factor of provisioning, depending on how many snaps you are keeping.
I hope this series has been useful – I’m really excited about this new technology pairing!
In this post we are going to configure a local consistency group within XtremIO, armed with our knowledge of the CG settings. I want to configure one snap per hour for 48 hours, 48 max snaps.
Because I’m working with local protection, I have to have the full featured licensing (/EX) instead of the basic (/SE) that only covers remote protection. Note: these licenses are different than normal /SE and /EX RP licenses! If you have an existing VNX with standard /SE, then XtremIO with /SE won’t do anything for you!
I have also already configured the system itself, so I’ve presented the 3GB repository volume, configured RP, and added this XtremIO cluster into RP.
All that’s left now is to present storage and protect! I’ve got a 100GB production LUN I want to protect. I have actually already presented this LUN to ESX, created a datastore, and created a very important 80GB Eager Zero Thick VM on it.
First thing’s first, I need to create a replica for my production LUN – this must be the exact same size as the production LUN, although again this is always my recommendation with RP anyway. I also need to create some journal volumes as well. Because this isn’t a distributed CG, I’ll be using the minimum 10GB sizing. Lucky for us creating volumes on XtremIO is easy peasy. Just a reminder – you must use 512 byte blocks instead of 4K, but you are likely using that already anyway due to lack of 4K support.
Next I need to map the volume. If you haven’t seen the new volume screen in XtremIO 4.0, it is a little different. Honestly I kind of like the old one which was a bit more visual but I’m sure I’ll come to love this one too. I select all 4 volumes and hit the Create/Modify Mapping button. Side note: notice that even though this is an Eager Zero’d VM, there is only 7.1MB used on the volume highlighted below. How? At first I thought this was the inline deduplication, but XtremIO does a lot of cool things, and one neat thing it does is discard all ful-zero block writes coming into the box! So EZTs don’t actually inflate your LUNs.
Next I choose the Recoverpoint Initiator group (the one that has ALL my RP initiators in it) and map the volume. LUN IDs have never really been that important when dealing with RP, although in remote protection it can be nice to try to keep the local and remote LUN IDs matching up. Trying to make both host LUN IDs and RP LUN ID match up is a really painful process, especially in larger environments, for (IMO) no real benefit. But if you want to take up that, I won’t stop you Sysyphus!
Notice I also get a warning because it recognizes that the Production LUN is already mapped to an existing ESX host. That’s OK though, because I know with RP this is just fine.
Alright now into Recoverpoint. Just like always I go into Protection and choose Protect Volumes.
These screens are going to look pretty familiar to you if you’ve used RP before. On this one, for me typically CG Name = LUN name or something like it, Production name is ProdCopy or something similar, and then choose your RPA cluster. Just like always, it is EXTREMELY important to choose the right source and destinations, especially with remote replication. RP will happily replicate a bunch of nothing into your production LUN if you get it backwards! I choose my prod LUN and then I hit modify policies.
In modify policy, like normal I choose the Host OS (BTW I’ll happily buy a beer for anyone who can really tell me what this setting does…I always set it but have no idea what bearing it really has!) and now I set the maximum number of snaps. This setting controls how many total snapshots the CG will maintain for the given copy. If you haven’t worked with RP before this can be a little confusing because this setting is for the “production copy” and then we’ll set the same setting for the “replica copy.” This allows you to have different settings in a failover situation, but most of the time I keep these identical to avoid confusion. Anywho, we want 48 max snaps so that’s what I enter.
I hit Next and now deal with the production journal. As usual I select that journal I created and then I hit modify policy.
More familiar settings here, and because I want a 48 hour protection window, that’s what I set. Again based on my experience this is an important setting if you only want to protect over a specific period of time…otherwise it will spread your snaps out over 30 days. Notice that snapshot consolidation is greyed out – you can’t even set it anymore. That’s because the new snapshot pruning policy has effectively taken its place!
After hitting next, now I choose the replica copy. Pretty standard fare here, but a couple of interesting items in the center – this is where you configure the snap settings. Notice again that there is no synchronous replication; instead you choose periodic or continuous snaps. In our case I choose periodic and a rate of one per 60 minutes. Again I’ll stress, especially in a remote situation it is really important to choose the right RPA cluster! Naming your LUNs with “replica” in the name helps here, since you can see all volume names in Recoverpoint.
In modify policies again we set that host OS and a max snap count of 48 (same thing we set on the production side). Note: don’t skip over the last part of this post where I show you that sometimes this setting doesn’t apply!
In case you haven’t seen the interface to choose a matching replica, it looks like this. You just choose the partner in the list at the bottom for every production LUN in the top pane. No different from normal RP.
Next, we choose the replica journal and modify policies.
Once again setting the required protection window of 48 hours like we did on the production side.
Next we get a summary screen. Because this is local it is kind of boring, but with remote replication I use this opportunity to again verify that I chose the production site and the remote site correctly.
After we finish up, the CG is displayed like normal, except it goes into “Snap Idle” when it isn’t doing anything active.
One thing I noticed the other day (and why I specifically chose these settings for this example) is that for some reason the replica copy policy settings aren’t getting set correctly sometimes. See here, right after I finished up this example the replica copy policy OS and max snaps aren’t what I specified. The production is fine. I’ll assume this is a bug until told otherwise, but just a reminder to go back through and verify these settings when you finish up. If they are wrong you can just fix them and apply.
Back in XtremIO, notice that the replica is now (more or less) the same size as the production volume as far as used space. Based on my testing this is because the data existed on the prod copy before I configured the CG. If I configure the CG on a blank LUN and then go in and do stuff, nothing happens on the replica LUN by default because it isn’t rolling like it use to. Go snaps!
I’ll let this run for a couple of days and then finish up with a production recovery and a summary.
In this post we are going to explore the new snap based protection with Recoverpoint and XtremIO. It is worth noting that some of this is based on my observations and testing, and encourage you to do the same in your environment. Take snaps, try to recover, etc. I used a relatively small sample size and testing criteria.
Also worth noting – you aren’t supposed to manually interact with the snapshots that RP is taking, and in fact you can’t even see them when logged in as admin. However if you log in as tech or rp_user, you can see them and optionally interact with them. But again remember, if you manually interact with these pieces you may cause issues in your environment! Leave this type of stuff to the testers or in test environments.
Snap Based Replication Behavior
So, snap based replication – what is is and how is it different?
Well standard Recoverpoint is pretty well documented but the idea is each write is:
- Split at the source array
- Sent to the remote array
- Finally applied to the journal volume there.
At a later time, this write will be applied to the replica LUN. So the journals contain a timeline of writes, and the replica LUN is somewhere alone that timeline at any given moment. No real clue where, but when you go to access an image the system (with direct access) the system will “roll” the replica using the write timeline to where ever you wanted.
Snap based replication is literally nothing like this. Instead it operates like this. Again I’m writing this based on my reading of the tech notes as well as what I “see” between RP and XtremIO. I write this from the perspective of a single source/replica combo but obviously you can have multiples just like always.
- Source LUN and Replica LUN (along with a single source and replica journal – remember no need to have large journals or even multiple journals) form a consistency group.
- On the source LUN, a snap is created that is labeled “SMP” – likely a reference to snapshot mount point, even though these don’t really exist on XtremIO. All snaps are just disks.
- On the DR side, the DR LUN also has a snap created that is SMP.
- On the DR side, two sub-snaps of the SMP are created called Volume##### (some incremental volume number). Presumably the first is the state of the LUN as it started with and the next is where the changes are headed. At this point if you look inside RP at the DR journal, you will see two snaps. Regardless…
- All changes (current contents of SMP) are sent across to the DR side. So at this point we’ve got Source LUN and Source SMP snap. We’ve also got DR LUN, DR SMP snap, and 2 x sub snaps.
- At some point (depending on how you’ve configured things) the system will:
- Take another prod side snap and DR snap, both Volume##### snaps. On the prod side, this snap is temporary because the differences between it and the prod SMP LUN represent the changes that need to be sent across.
- These changes are sent across and injected into the DR snapshot, which is your newest snapshot for recovery.
- Once this is complete, the temporary snap on the source is merged into the SMP snap, which now represents the state of the source LUN from last replication.
Now the source SMP and the latest snap are identical.
This process repeats indefinitely and represents your ongoing protection .
So clearly a departure from what we are used to. Because all changes are stored in snapshots, no journal space is necessary for storing writes. And there is also no need to keep rolling the replica either, because the recovery points on RP are in-memory snapshots on XtremIO (pointer based) which can be promoted or merged at any time near instantaneously. I self-confirmed no replica rolling by:
- Configuring a CG on a blank LUN and letting replication start rolling through snaps.
- Mounting prod LUN on vSphere and create VMFS datastore, noting some activity in the snaps.
- Waited a few more replication cycles
- Paused CG
- Unmounted/unmapped prod LUN
- Manually mapped replica LUN
- Mounted/attached replica LUN in vSphere, but it does not contain a VMFS file system. This is just a raw LUN, indicating that there is no more replica rolling in the background.
- Unmounted/unmapped replica LUN
- Enabled image access on newest snapshot
- Mapped/mounted/attached replica LUN in vSphere. Now the VMFS file system is there.
- Detached replica and disabled image access.
- Reattached replica LUN, VMFS file system is still there. So it didn’t try to restore the “nothing” that was in the LUN to begin with since there is no good reason to do that.
One thing I didn’t test is whether the snaps get merged into the replica LUN as they roll off the image list. I don’t think this is the case – I think they are actually merged into the DR side SMP LUN, though I haven’t confirmed.
But either way, again, very cool how this new functionality leverages XtremIO snaps for efficient replication.
Another nice change is that image access no longer uses the journal, because essentially all changes are snap based and stored in the XtremIO pool. So no worries about long term image access and filling up the log.
I did image access on a raw LUN and presented to vSphere. Created a new datastore and deployed an EZT VMDK. In the RP GUI, there was no extra activity on the journal side.
Interestingly, the “undo writes” button still works. In this case I unmounted that LUN from vSphere and clicked undo writes. When I attempted to remount/readd, there was no datastore on it.
Consistency Group Snapshot Behavior
When you configure a consistency group, you will configure a few parameters related to your protection. The first is Maximum Number of Snapshots. This is the total amount of snapshots that consistency group will retain, and goes up to 500. Don’t forget that there is a per-XMS limitation of 8,192 total volumes + snapshots! If you configure 500 snaps per group then you’ll probably run out quickly and won’t even be able to create new LUNs on XtremIO.
The other parameter you’ll configure is the type of protection you want. There is no synchronous mode with RP+XtremIO. Instead you choose Continuous which essentially creates a new snap as soon as the previous one is done transferring, or Periodic which will take snaps every X minutes.
With Continuous there isn’t really anything else to configure. You can configure an RPO in minutes, but this is allegedly just an alerting mechanism.
With Periodic, you do tell it how often to take the snaps. You can configure down to a per minute snapshot if you want.
Alright, so now the weirdness – the snapshot pruning policy. The snapshot pruning policy is designed to give you a nice “spread” of snapshots. This is listed in the whitepaper as follows (these percentages are not currently adjustable):
Age of snapshots // Percentage of total
- 0–2 hours // 40%
- 2–24 hours // 30%
- 1–2 days // 20%
- 2–4 days // 5%
- 5–30 days // 5%
This is kind of helpful, except they don’t really tell you how or when this policy is applied. In my testing, here is what I believe to be true.
- Unlike previous versions, the “Required Protection Window” setting actively alters what snapshots are removed. In classic RP, required protection window was simply an alerting mechanism. Now it appears that if you configure a required protection window of Z hours with X snapshots, most of the time the system will work to stagger those out so you will have X snapshots distributed throughout your Z hours.
- For instance, if you told the system I want periodic snaps every minute, 10 maximum snapshot count, and a required protection window of 5 hours, it will start out by taking one snap a minute for 10 minutes. After that, it will begin deleting snaps in the middle but preserving the first ones it took. Here I still have 2 of the first snaps it took, but a lot of intermediary ones have been purged. It will continue this process until you get to the 5 hour mark, when it starts purging the oldest snap. So you will end up with a 5 hour rolling protection window at the end of the day. Same thing if you said 12 hour, or 1 day, or 1 week, etc.
- If you reduce your Required Protection Window, the system will immediately purge snapshots. So for instance if I have my 5 hour window as in my previous example, with 5 hours worth of snaps, and I reduce my Required Protection Window to 3 hours, any snaps past 3 hours are immediately deleted.
- By default (again, I believe this to be true), a consistency group will have an unwritten Required Protection Window of 1 month. I noticed while tinkering around that if a CG doesn’t have a Protection Window set, it looks like it will try to go for 30 days worth of snaps. And sometimes (in the midst of testing copies and other things) it actually set a 30 day window on the CG without my interaction.
- If the protection window is 1 or 2 hours, no snapshot pruning is done. This kind of matches up with the stated pruning policy which starts to delineate after 2 hours. But e.g. if I configure a CG with 10 snap max, 1 per minute, and a 1 or 2 hour required protection window, then my actual recovery will only ever be 10 minutes long and I will never meet my specified requirements. After 10 snaps exist, the newest snap always replaced the old one. BUT! If I set my Required Protection Window to 3+ hrs, then it starts doing the odd pruning/purging so that my total protection window is met.
- The pruning behavior seems to be the same whether you have Periodic snaps or Continuous snaps in place.
Again I found this to be a little odd and hope there is some clearer documentation in the future, but in the meantime this is my experience.