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tuomassalo
Asked: 2010-06-11 12:07:36 +0800 CST

Best practices for thin-provisioning Linux servers (on VMware)

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I have a setup of about 20 Linux machines, each with about 30-150 gigabytes of customer data. Probably the size of data will grow significantly faster on some machines than others. These are virtual machines on a VMware vSphere cluster. The disk images are stored on a SAN system.

I'm trying to find a solution that would use disk space sparingly, while still allowing for easy growing of individual machines.

In theory, I would just create big disks for each machine and use thin provisioning. Each disk would grow as needed. However, it seems that a 500 GB ext3 filesystem with only 50 GB of data and quite a low number of writes still easily grows the disk image to eg. 250 GB over time. Or maybe I'm doing something wrong here? (I was surprised how little I found on the subject with Google. BTW, there's even no thin-provisioning tag on serverfault.com.)

Currently I'm planning to create big, thin-provisioned disks - but with a small LVM volume on them. For example: a 100 GB volume on a 500 GB disk. That way I could more easily grow the LVM volume and the filesystem size as needed, even online.

Now for the actual question:

Are there better ways to do this? (that is, to grow data size as needed without downtime.)

Possible solutions include:

  • Using a thin-provisioning friendly filesystem that tries to occupy the same spots over and over again, thus not growing the image size.

  • Finding an easy method of reclaiming free space on the partition (re-thinning?)

  • Something else?

A bonus question: If I go with my current plan, would you recommend creating partitions on the disks (pvcreate /dev/sdX1 vs pvcreate /dev/sdX)? I think it's against conventions to use raw disks without partitions, but it would make it a bit easier to grow the disks, if that is ever needed. This is all just a matter of taste, right?

linux lvm

5 Answers

  1. Best Answer

    If I understand thin provisioning correctly then it could really cause problems if you aren't monitoring your VMFS filesystems growth closely and allow your VMDKs to fill up your VMFS volumes. You've seen in your testing that thin provisioned disks tend to grow to fill their available space quickly and that they cannot reclaim space that may be free inside the OS.

    The other option is creating sufficiently sized VMDK files to handle your current usage and expected spikes in growth and just add more VMDK files as your application data usage grows. New VMDK files can be added live to a VM, you just have to rescan (echo "- - -" > /sys/class/scsi_host/host?/scan). You can partition the new disk, add it to your LVM and extend the filesystem all live. This way you are always aware how much space is allocated to each of the VMs and you can't accidently run your VMFS out of space from inside a guest.

    As far as whether to partition or not if the disk is only going to be used by LVM, I always partition. Partitioning the disk prevents any warnings about bogus partition tables from coming up when the machine boots and makes it clear that the disk is allocated. It's a bit of voodoo but I also make sure to start the partition at 64 to help make sure the partition and filesystem is block aligned with the underlying storage. It's hard to detect and categorize as you usually don't have something to easily compare against but if the OS filesystem isn't aligned properly with the underlying storage then you can end up with extra IOPS required to service requests which cross block boundaries on the underlying storage.

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  2. The best suggestion i can think of is to create an LVM setup with physical volumes, volume groups and logical volumes, then mount those logical volumes as your VM's filesystem via iSCSI.

    This will enable you to resize the logical volume and then all you'd need to do after that is restart your iscsi daemon and virtual machine software and check that it has the new size parameters and then resize the guest's filesystem to match.

    The partitioning would work as with a standard hdd, as thats how the LV would appear to the virtual machine guest.

    Edit: never mind this, I got the false impression you were running vmware under linux, not linux under vmware.

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  3. I dont know how it works in VMware but Redhat RHEV-M/RHEV-H its possible and it support RHEL 4.8 to 5.X and same time for win 2k3 R2 and win 2k8. For more info http://studyhat.blogspot.com/2010/05/rhev-for-servers-22-installation-and.html

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  4. Different suggestion: setup a fileserver which will hold all your userdata.

    That fileserver of course should be managed with LVM for your online capacity management purposes.

    If you wish to, you can create a ha-setup (drbd + heartbeat) with one copy of your fileserver in the SAN and a second copy outside or alike. (for the paranoids)

    Since your clients are linux based you can use NFS which is said to be faster than Samba. A FileServer will allow you a centralized backup strategy as well as a centralized storage usage monitoring. And in terms of your thin provisioning question:

    Create your LVM setup as you intended (in your example 100gb LV on a 500gb PV, just change the numbers matching the sum of storage your VMs need). Expand this LV when needed, as you planned to do in each VM separately. But just do it ONCE on your fileserver. Each time a VM reduces its storage usage that space will become available for all your VMs ;-)

    Use quotas on your fileserver if needed or desired to prevent a single VM from filling up your fileserver.

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  5. Are you able to expand a volume without shutting down the virtual machine using it? I commonly do this, but I don't know what your storage setup is like or how VMware might complicate things. If you can, what I would do is this

    1) Don't thin provision. Make each volume the size you think you need. 2) Don't use partitions. Make every filesystem its own volume. 3) Monitor filesystem growth so you can resize volumes proactively.

    When it comes time to grow a volume

    1) On your SAN or in VMware, do whatever you need to expand the volume. 2) In linux, run echo 1 > /sys/block/EXAMPLE/device/rescan, where EXAMPLE is the device names under /sys/block/. 3) In linux, run resize2fs /dev/EXAMPLE, where EXAMPLE is the device name under /dev.

    This approach is working well for me. I did consider your approach with thin provisioning and LVM, and I think it would work as well.

    If you do decide to go the LVM route, I recommend not partitioning the disks. As you said, not having a partition table makes it easier to grow the virtual disk. With no partition table, you can just run pvresize and linux will recognize that the physical volume has grown. With a partition table, you have to unmount any filesystems, delete the partiton table, recreate the partition table, then run pvresize. That's a lot more work, and it requires downtime.

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