ZFS, Zvol, iSCSI and windows

[Karli Sjöberg]

fre 2015-09-25 klockan 11:08 +0200 skrev Willem Jan Withagen:
> Hi,
> 
> Because of the Network Video Recorder (on windows) we use only likes 
> "real" disks, and not SMB disks. We started using ZVOLs which are 
> exported thru iSCSI/ctld....
> 
> And that works really well, so there all thumbs up for this combo.
> 
> However..... (you knew that was coming)
> I do have some questions, and hope somebody can share some insights.
> 
> 1)
> This is a ZFS question.
> 
> I have created a ZVOL with:
> 	zfs create -s -V 5T zfsraid/nvr2
> 
> Looking at the disk usage in 'zfs get all':
> NAME          PROPERTY              VALUE                  SOURCE
> zfsraid/nvr2  used                  9.97T                  -
> zfsraid/nvr2  available             438G                   -
> zfsraid/nvr2  referenced            9.97T                  -
> zfsraid/nvr2  compressratio         1.00x                  -
> zfsraid/nvr2  reservation           none                   default
> zfsraid/nvr2  volsize               5T                     local
> zfsraid/nvr2  volblocksize          8K                     -
> zfsraid/nvr2  checksum              on                     default
> zfsraid/nvr2  compression           lz4                    default
> zfsraid/nvr2  primarycache          all                    default
> zfsraid/nvr2  secondarycache        all                    default
> zfsraid/nvr2  usedbysnapshots       0                      -
> zfsraid/nvr2  usedbydataset         9.97T                  -
> zfsraid/nvr2  usedbychildren        0                      -
> zfsraid/nvr2  usedbyrefreservation  0                      -
> zfsraid/nvr2  sync                  standard               default
> zfsraid/nvr2  written               9.97T                  -
> zfsraid/nvr2  logicalused           4.97T                  -
> zfsraid/nvr2  logicalreferenced     4.97T                  -
> zfsraid/nvr2  volmode               default                default
> 
> And what sort of "worries" me is that it seems that this volume is using 
> twice the amount of diskspace it is offering as ZVOL?
> 
> a) Is this really true?
> b) Should I have done something different to not waste so much overhead?
> 
> Note that the compression rate is 1.00x, which is of course due to 
> writing h264 media streams that do not compress at all. But is the 
> parents default, and I haven't turned it off.
> 
> 2)
> The second one might be more a Windows question, but anyways.
> 
> The export is that same 5T ZVOL, plain create with
> 	zfs create -s -V 5T zfsraid/nvr2
> Under Windows I used the regular stuff to format the dis with GPT and 
> NTFS with 8k segments. (matching the ZVOL blocksize)
> 
> Upon reboot FreeBSD notices the following:
> GEOM_PART: partition 1 on (zvol/zfsraid/nvr2, GPT) is not aligned on
> 	8192 bytes	
> By itself not technical problem. But the warning does hint that the 
> alignment is thus, that performance might suffer. the backing disks are 
> WD REDs which are 4K sectors.... So the misalignment could cause too 
> many read/writes for writing.
> 
> Does anybody have a clue as how to get Windows to do the aligning correctly?
> 
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Is it this effect you are seeing:
https://forums.freebsd.org/threads/mitigating-4k-disk-issues-on-raidz.37365/

/K