Re: [Scst-devel] Honoring SYNC requests in combination with ZFS

[Sietse v. Z. <si...@wi...>]

Yes, because iSCSI is well, SCSI and NFS is NFS. The SCSI protocol set does not contain any sort of SYNC_WRITE command, whereas the NFS protocol does. This is very important to keep in mind when designing a storage infrastructure based on any SCSI protocol set, whether it being locally attached or remotely. And even with NFS, you have to pay attention to what is actually at the back-end, which is well SCSI most likely.



And like I said before even a UPS will not cut it when disk write cache is used, there will still be a very minor chance of the disk purging its cache upon a reset before writing it to its magnetic platters.



Protected cache is the only cache you can use if sync writes is what you require, and then only with redundant memory. Or you will still suffer data loss in case the memory of your RAID controller dies. Good luck with the battery in that case. And even with redundant memory, what would you do in case the whole controller catches fire because of the battery overheating....



volatile cache == increased risks to your data. PERIOD.



-Sietse





________________________________
From: T.C. Ferguson [tc...@gm...]
Sent: Tuesday, March 04, 2014 16:26
To: Sietse van Zanen
Cc: Vladislav Bolkhovitin; scs...@li...
Subject: Re: [Scst-devel] Honoring SYNC requests in combination with ZFS

VMware does not issue a sync request with every single I/O when using iSCSI. This is in contrast to its use of NFS -- where it does issue every single io with a sync request.

We see this a lot in testing...people wonder why iscsi is so much faster than nfs using their homegrown solutions (most production nfs products don't honor sync as they use battery backed nvram or otherwise to protect writes).

You can use write_through = 1 to force SCST to write out it's data...then you can setup ZFS to sync=always to force data to zil.

Or you can go get a decent ups and call it a day.



On Sun, Mar 2, 2014 at 4:58 AM, Sietse van Zanen <si...@wi...<mailto:si...@wi...>> wrote:
I don't believe so. ZFS has its own caching mechanism, so this setting will not have any effect, just like o_direct. The effect is achieved by the zfs set sync=always setting for the filesystem or zvol..

-Sietse

-----Original Message-----
From: Vladislav Bolkhovitin [mailto:vs...@vl...<mailto:vs...@vl...>]
Sent: Friday, February 28, 2014 22:50
To: Sietse van Zanen
Cc: Thijs Cramer; scs...@li...<mailto:scs...@li...>
Subject: Re: [Scst-devel] Honoring SYNC requests in combination with ZFS

Sietse van Zanen, on 02/28/2014 02:57 AM wrote:
> You have to do 3 things.
>
>
>
> 1. Set nv_cache to 0 on the scst side

You mean write_through in 1 here, I believe.

Vlad

> 2. zfs set sync=always for the filesystem or zvol you have the file on.
>
> 3. Use ZIL or your performance will be terrible, definitely with SATA
>
>
>
> -Sietse
>
>
>
> ________________________________
> From: Thijs Cramer [thi...@gm...<mailto:thi...@gm...>]
> Sent: Tuesday, February 25, 2014 08:59
> To: scs...@li...<mailto:scs...@li...>
> Subject: [Scst-devel] Honoring SYNC requests in combination with ZFS
>
> Guys,
>
> Is there a definitive answer on whether we can use the fileio handler in combination with ZFSonLinux?
> I'm seeing extreme performance statistics > 18.000 IOPS over iSCSI when using ZFSonLinx on 4*4TB drives with sync=standard. The disks are exported over iSCSI through a ZVOL and I've defined a fileio handler in SCST. The iSCSI volume is exported to an ESXi cluster.
>
> I think this means that SCST doesn't honor the SYNC requests from ESXi.
>
> I know the linux page cache is involved when using the fileio handler, but i thought sync requests would still be honored? Am i mistaking?
>
> - Thijs
>
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T.C. Ferguson
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