iSCSI Device Extent vs File Extents

[Austin Downing]

Previous to FreeNAS 9.x.x I was under the impression that it was not possible to setup a ZFS volume and create a iSCSI share with a Extent pointing to that zvol. Instead you needed to create a File Extent for a zvol based iSCSI share.

Correct me if I am wrong but it appears that with the FreeNAS 9.x.x, it is now possible to created a device extent based on a zvol. Because of that I am wondering if I upgrade to 9.1.1 from 8.3.2 and create a device extent instead of a file extent will I see a performance boost?

If so should it be substantial or a very minor boost in performance. Will a device extent behave better than a file extent in the long run because of the COW of ZFS?

 

[cyberjock]

Previous to FreeNAS 9.x.x I was under the impression that it was not possible to setup a ZFS volume and create a iSCSI share with a Extent pointing to that zvol. Instead you needed to create a File Extent for a zvol based iSCSI share.

Correct me if I am wrong but it appears that with the FreeNAS 9.x.x, it is now possible to created a device extent based on a zvol. Because of that I am wondering if I upgrade to 9.1.1 from 8.3.2 and create a device extent instead of a file extent will I see a performance boost?

There are a boatload of factors that can make performance increase or decrease. The only way to know if it works for you is to try it(or do a lot of theoretical research on what affects performance) and figure out if it works for your work load and your situation.

If so should it be substantial or a very minor boost in performance. Will a device extent behave better than a file extent in the long run because of the COW of ZFS?

Uh, you don't understand. The CoW properly is for a zpool. Anything on a ZFS file system is CoW. There is no disabling this that I know of. So every single file, every single zvol, everything, is CoW. If CoW is not working out for you the alternative is to use UFS. Note that UFS doesn't work for everything(for examples, jails won't work on UFS)

 

[Austin Downing]

I guess my question is, can I do device extent based on my ZFS volumes?

 

[pirateghost]

I guess my question is, can I do device extent based on my ZFS volumes?

I use device extents on my volume. I don't use file extents at all

 

[dlavigne]

Both types of extents have been available in both the 8.x and 9.x series (though how they appeared in the tree differs very slightly between versions). From http://doc.freenas.org/index.php/ISCSI#Extents:

In theory, a zvol and a file extent should have identical performance. In practice, a file extent outperforms in reads/writes but this is only noticeable at 10 GB Ethernet speeds or higher. For high performance, file extents are recommended at this time. Future changes to FreeBSD's zvol code will increase its performance.

This is specific to the FreeBSD implementation which FreeNAS is based upon.

 

[enemy85]

Sorry to re-open an old post, but it's about iSCSI device extent so i thought it was useless to open a new thread for it...

I settled down a zvol (I access via iSCSI device extent) which I use to backup my laptop and other Pc, and everything works perfectly, now the question is: is there a way to access the files are on this zvol apart from iSCSI? i mean, can i see in some way the files on it through the shell?

 

[Juan Pablo Abuyeres]

It depends on what fs are you using. If it is VMFS, take a look at https://bugs.freenas.org/issues/5224 and if it is another fs, supported in FreeBSD, you may mount it directly on your FreeNAS box. Be careful not to mount a non-Cluster-Aware fs more than once at the same time, or you may corrupt data.