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ZFS Administration, Part XIV- ZVOLS

Table of Contents

Zpool Administration ZFS Administration Appendices
0. Install ZFS on Debian GNU/Linux 9. Copy-on-write A. Visualizing The ZFS Intent Log (ZIL)
1. VDEVs 10. Creating Filesystems B. Using USB Drives
2. RAIDZ 11. Compression and Deduplication C. Why You Should Use ECC RAM
3. The ZFS Intent Log (ZIL) 12. Snapshots and Clones D. The True Cost Of Deduplication
4. The Adjustable Replacement Cache (ARC) 13. Sending and Receiving Filesystems
5. Exporting and Importing Storage Pools 14. ZVOLs
6. Scrub and Resilver 15. iSCSI, NFS and Samba
7. Getting and Setting Properties 16. Getting and Setting Properties
8. Best Practices and Caveats 17. Best Practices and Caveats

What is a ZVOL?

A ZVOL is a "ZFS volume" that has been exported to the system as a block device. So far, when dealing with the ZFS filesystem, other than creating our pool, we haven't dealt with block devices at all, even when mounting the datasets. It's almost like ZFS is behaving like a userspace application more than a filesystem. I mean, on GNU/Linux, when working with filesystems, you're constantly working with block devices, whether they be full disks, partitions, RAID arrays or logical volumes. Yet somehow, we've managed to escape all that with ZFS. Well, not any longer. Now we get our hands dirty with ZVOLs.

A ZVOL is a ZFS block device that resides in your storage pool. This means that the single block device gets to take advantage of your underlying RAID array, such as mirrors or RAID-Z. It gets to take advantage of the copy-on-write benefits, such as snapshots. It gets to take advantage of online scrubbing, compression and data deduplication. It gets to take advantage of the ZIL and ARC. Because it's a legitimate block device, you can do some very interesting things with your ZVOL. We'll look at three of them here- swap, ext4, and VM storage. First, we need to learn how to create a ZVOL.

Creating a ZVOL

To create a ZVOL, we use the "-V" switch with our "zfs create" command, and give it a size. For example, if I wanted to create a 1 GB ZVOL, I could issue the following command. Notice further that there are a couple new symlinks that exist in /dev/zvol/tank/ and /dev/tank/ which points to a new block device in /dev/:

# zfs create -V 1G tank/disk1
# ls -l /dev/zvol/tank/disk1
lrwxrwxrwx 1 root root 11 Dec 20 22:10 /dev/zvol/tank/disk1 -> ../../zd144
# ls -l /dev/tank/disk1
lrwxrwxrwx 1 root root 8 Dec 20 22:10 /dev/tank/disk1 -> ../zd144

Because this is a full fledged, 100% bona fide block device that is 1 GB in size, we can do anything with it that we would do with any other block device, and we get all the benefits of ZFS underneath. Plus, creating a ZVOL is near instantaneous, regardless of size. Now, I could create a block device with GNU/Linux from a file on the filesystem. For example, if running ext4, I can create a 1 GB file, then make a block device out of it as follows:

# fallocate -l 1G /tmp/file.img
# losetup /dev/loop0 /tmp/file.img

I now have the block device /dev/loop0 that represents my 1 GB file. Just as with any other block device, I can format it, add it to swap, etc. But it's not as elegant, and it has severe limitations. First off, by default you only have 8 loopback devices for your exported block devices. You can change this number, however. With ZFS, you can create 2^64 ZVOLs by default. Also, it requires a preallocated image, on top of your filesystem. So, you are managing three layers of data: the block device, the file, and the blocks on the filesystem. With ZVOLs, the block device is exported right off the storage pool, just like any other dataset.

Let's look at some things we can do with this ZVOL.

Swap on a ZVOL

Personally, I'm not a big fan of swap. I understand that it's a physical extension of RAM, but swap is only used when RAM fills, spilling the cache. If this is happening regularly and consistently, then you should probably look into getting more RAM. It can act as part of a healthy system, keeping RAM dedicated to what the kernel actively needs. But, when active RAM starts spilling over to swap, then you have "the swap of death", as your disks thrash, trying to keep up with the demands of the kernel. So, depending on your system and needs, you may or may not need swap.

First, let's create 1 GB block device for our swap. We'll call the dataset "tank/swap" to make it easy to identify its intention. Before we begin, let's check out how much swap we currently have on our system with the "free" command:

# free
             total       used       free     shared    buffers     cached
Mem:      12327288    8637124    3690164          0     175264    1276812
-/+ buffers/cache:    7185048    5142240
Swap:            0          0          0

In this case, we do not have any swap enabled. So, let's create 1 GB of swap on a ZVOL, and add it to the kernel:

# zfs create -V 1G tank/swap
# mkswap /dev/zvol/tank/swap
# swapon /dev/zvol/tank/swap
# free
             total       used       free     shared    buffers     cached
Mem:      12327288    8667492    3659796          0     175268    1276804
-/+ buffers/cache:    7215420    5111868
Swap:      1048572          0    1048572

It worked! We have a legitimate Linux kernel swap device on top of ZFS. Sweet. As is typical with swap devices, they don't have a mountpoint. They are either enabled, or disabled, and this swap device is no different.

Ext4 on a ZVOL

This may sound wacky, but you could put another filesystem, and mount it, on top of a ZVOL. In other words, you could have an ext4 formatted ZVOL and mounted to /mnt. You could even partition your ZVOL, and put multiple filesystems on it. Let's do that!

# zfs create -V 100G tank/ext4
# fdisk /dev/tank/ext4
( follow the prompts to create 2 partitions- the first 1 GB in size, the second to fill the rest )
# fdisk -l /dev/tank/ext4

Disk /dev/tank/ext4: 107.4 GB, 107374182400 bytes
16 heads, 63 sectors/track, 208050 cylinders, total 209715200 sectors
Units = sectors of 1 * 512 = 512 bytes
Sector size (logical/physical): 512 bytes / 8192 bytes
I/O size (minimum/optimal): 8192 bytes / 8192 bytes
Disk identifier: 0x000a0d54

          Device Boot      Start         End      Blocks   Id  System
/dev/tank/ext4p1            2048     2099199     1048576   83  Linux
/dev/tank/ext4p2         2099200   209715199   103808000   83  Linux

Let's create some filesystems, and mount them:

# mkfs.ext4 /dev/zd0p1
# mkfs.ext4 /dev/zd0p2
# mkdir /mnt/zd0p{1,2}
# mount /dev/zd0p1 /mnt/zd0p1
# mount /dev/zd0p2 /mnt/zd0p2

Enable compression on the ZVOL, copy over some data, then take a snapshot:

# zfs set compression=lzjb pool/ext4
# tar -cf /mnt/zd0p1/files.tar /etc/
# tar -cf /mnt/zd0p2/files.tar /etc /var/log/
# zfs snapshot tank/ext4@001

You probably didn't notice, but you just enabled transparent compression and took a snapshot of your ext4 filesystem. These are two things you can't do with ext4 natively. You also have all the benefits of ZFS that ext4 normally couldn't give you. So, now you regularly snapshot your data, you perform online scrubs, and send it offsite for backup. Most importantly, your data is consistent.

ZVOL storage for VMs

Lastly, you can use these block devices as the backend storage for VMs. It's not uncommon to create logical volume block devices as the backend for VM storage. After having the block device available for Qemu, you attach the block device to the virtual machine, and from its perspective, you have a "/dev/vda" or "/dev/sda" depending on the setup.

If using libvirt, you would have a /etc/libvirt/qemu/vm.xml file. In that file, you could have the following, where "/dev/zd0" is the ZVOL block device:

<disk type='block' device='disk'>
  <driver name='qemu' type='raw' cache='none'/>
  <source dev='/dev/zd0'/>
  <target dev='vda' bus='virtio'/>
  <alias name='virtio-disk0'/>
  <address type='pci' domain='0x0000' bus='0x00' slot='0x05' function='0x0'/>

At this point, your VM gets all the ZFS benefits underneath, such as snapshots, compression, deduplication, data integrity, drive redundancy, etc.


ZVOLs are a great way to get to block devices quickly while taking advantage of all of the underlying ZFS features. Using the ZVOLs as the VM backing storage is especially attractive. However, I should note that when using ZVOLs, you cannot replicate them across a cluster. ZFS is not a clustered filesystem. If you want data replication across a cluster, then you should not use ZVOLs, and use file images for your VM backing storage instead. Other than that, you get all of the amazing benefits of ZFS that we have been blogging about up to this point, and beyond, for whatever data resides on your ZVOL.

{ 10 } Comments

  1. Ahmed Kamal | July 22, 2013 at 2:56 pm | Permalink

    When you put ext4 on top of a ZVOL, and snapshot it .. You say it's "consistent" I guess it's only crash-consistent .. There is no FS/ZVOL integration to ensure better consistency, right

  2. Aaron Toponce | August 7, 2013 at 10:08 am | Permalink

    Not sure what you're saying. When you put ext4 on top of a ZVOL, ext4 is just a standard run-of-the-mill application wishing to stare data on ZFS just as much as anything else. So, the data is pooled into a TXG just as anything else. TXGs are flushed in sequential order to the ZIL. The contents of the ZIL are flushed to disk synchronously. So, the data is always consistent.

    Suppose you have a VM that is using that ZVOL for its storage. Suppose further that your VM crashes. At worst case, the ext4 journel is not closed. So, at next boot, you will be forced to fsck(8) the disk. What's important to know, is that the data on ZFS is still consistent, even if ext4 may have lost some data as a result of the crash. In other words, closing the journal did not happen before the rest of the data blocks were flushed to disk.

  3. Jack Relish | August 26, 2013 at 3:25 pm | Permalink

    Excellent guide. I was wondering if you had any insights on mounting partitions that exist on a ZVOL?

    For example, lets say that I have a ZVOL /dev/tank/vm0, which was used as the root device for a VM. At some point, the VM breaks or for whatever other reason I want to be able to access the contents of it's filysystem. Is it possible to expose the internals of the ZVOL? I'm sure that it could be done manually an tediously by getting the start offset of the partition and then mounting it in the same way you would a raw image file, but if there is a slicker way to do so that would be incredible.

  4. Zyon | March 10, 2014 at 7:17 pm | Permalink

    When you say "you cannot replicate them across a cluster", that means I cannot use DRBD over ZVOL?

  5. Andy | April 12, 2014 at 6:58 am | Permalink


    kpartx is probably what you'd need for getting at partitions within a ZVOL holding a VM's disk. Certainly it works for images taken from entire real disks using dd.


  6. sammand | June 3, 2014 at 9:26 pm | Permalink


    Yes ZVOLs can be replicated using DRBD and we support it in ZBOSS Linux distribution.
    You are free to check it out

  7. cbu | October 20, 2014 at 10:31 am | Permalink

    First thank for this incredible tutorial.
    I created a zvol and I am trying to attach it to a VM but I could not. Every time that I try to create a new Block device Storage Pool from virt-manager I get:

    RuntimeError: Could not start storage pool: internal error: Child process (/usr/bin/mount -t auto /dev/zvol/tank/rhel/disk1 /var/lib/libvirt/images/sss) unexpected exit status 32: mount: /dev/zd0 is write-protected, mounting read-only
    mount: unknown filesystem type '(null)'

    How can I get a block device available for Qemu? Thanks!

    P.S: I am using centOS 7 and zfs-0.6.3-1.1

  8. Mark | February 17, 2015 at 1:43 pm | Permalink

    Since ZFS is so vulnerable to fragmentation, would BTRFS on ZVOLs be a working combination? You'd miss the shared space usage between datasets, but you'd gain the advantages of the now stable BTRFS filesystem, while maintaining the reliability of RAID using ZFS. BTRFS RAID is still unstable, but it can defragment. And one can grow a ZVOL and BTRFS when needed. BTRFS is still evolving, while ZFS is stable, but Oracle will likely never release the new code (goodbye encryption) and the v5000 code isn't evolving much either. Wonder where compression would be more effective though. What's your opinion?

  9. Luca | September 8, 2015 at 5:49 am | Permalink

    Thanks for these very interesting articles on zfs!
    I want to set up some VM with Xen on ZFS and it is not clear to me which is the best solution for guest disk images: on LVM I use one LV and, when it fills up, I have to extend it. On ZFS I suppose I must create a ZVOL of some extent but what do to when is full? What is the smartest way to manage the VMs on ZFS?

  10. John Naggets | October 10, 2015 at 5:57 am | Permalink

    How do you create a ZVOL using 100% of the available size with its -V option?

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