Disk space usage with NTFS formatted iSCSI ZVOL?

[Hexland]

There's something I'm not understanding in the whole process chain of setting up an iSCSI Extent on a ZVOL, and presenting it to Windows as an iSCSI target and formatting it as NTFS...

So, I've got it all set up...
FreeNAS 9.2.1.7
10Tb ZPOOL with an 8Tb ZVOL exported as an iSCSI Extent
Windows Server 2012, with the ZVOL mounted as a drive, and formatted to NTFS

Out of 10Tb, I created an 8Tb ZVOL (leaving 20% space to avoid ZFS space issues).
I formatted the 8Tb ZVOL as NTFS and copied the contents of my Windows Server to it.
I got about 5Tb copied and I started to get 'Out of space' warnings from FreeNAS...

Now, I suspect it's a block size issue - everything is currently set to default (I didn't change any block sizes) - but I don't really understand what's going on.

Can someone explain to me why I'm only getting 50% usable capacity?

Thx

 

[Ericloewe]

You should not use up more than 50% of available space when using iSCSI, to avoid excessive fragmentation.

 

[cyberjock]

Well, few things...

1. If you are using iSCSI on ZFS you should try to keep your pool only 50-70% full.
2. It is very possible to be a block size problem. The more you right the more small blocks you'll have and the more space that the given size will actually consume.
3. Do not confuse Tb and TB. They mean two totally different things.

 

[diehard]

Have you deleted anything? iSCSI unmap is not supported with istgt. Anything you delete wont show as free space on the zvol. "Experimental mode" uses kernel target which supports this but i have not read up on what initiators will issue the command..

 

[mav@]

I successfully tested UNMAP from Windows 2012 to the experimental iSCSI target version which should be present in FreeNAS 9.2.1.8 and 9.3. FreeNAS 9.2.1.7 technically already has UNMAP support, but due to one small bug Windows was not using it. Now it should be fixed.

 

[cyberjock]

I successfully tested UNMAP from Windows 2012 to the experimental iSCSI target version which should be present in FreeNAS 9.2.1.8 and 9.3. FreeNAS 9.2.1.7 technically already has UNMAP support, but due to one small bug Windows was not using it. Now it should be fixed.

Is there a workaround for lowly 9.2.1.7 users?

 

[mav@]

Is there a workaround for lowly 9.2.1.7 users?

I don't know about such. But AFAIK VMWare still uses UNMAP. It seems bug affects only Windows.

 

[Alphonso]

Sorry to jump in on this conversation but I have/had the same problem. Running a 800G iscsi extent which i use for daily backups in our office. A lot of stuff get deleted daily as the backups are only for short term (keeps a weeks worth of backups). I managed to rescue my extent after it got full by using sdelete to write zero blocks to the free space on the windows machine. Is it a decent idea to run sdelete on a weekly task schedule until the UNMAP issue is resolved or are there risks involved in overusing this utility?

 

[mav@]

I have no experience with this program, but I think, if ZVOL has compression enabled, writing zeroes (sdelete manual mentions some "pattern", but I hope it can be switched to zeroes) into free blocks should give effect close to using UNMAP. ZFS has special case for compression of zero-only blocks, when they don't consume any real space. Metadata are still updated, so I guess doing it too often in combination with snapshots may consume some space for a multiple copies of metadata. Though I think it is better then nothing