On Aug 4, 2010, at 12:04 PM, Roch <roch.bourbonn...@sun.com> wrote: > > Ross Walker writes: >> On Aug 4, 2010, at 9:20 AM, Roch <roch.bourbonn...@sun.com> wrote: >> >>> >>> >>> Ross Asks: >>> So on that note, ZFS should disable the disks' write cache, >>> not enable them despite ZFS's COW properties because it >>> should be resilient. >>> >>> No, because ZFS builds resiliency on top of unreliable parts. it's able to >>> deal >>> with contained failures (lost state) of the disk write cache. >>> >>> It can then export LUNS that have WC enabled or >>> disabled. But if we enable the WC on the exported LUNS, then >>> the consumer of these LUNS must be able to say the same. >>> The discussion at that level then needs to focus on failure groups. >>> >>> >>> Ross also Said : >>> I asked this question earlier, but got no answer: while an >>> iSCSI target is presented WCE does it respect the flush >>> command? >>> >>> Yes. I would like to say "obviously" but it's been anything >>> but. >> >> Sorry to probe further, but can you expand on but... >> >> Just if we had a bunch of zvols exported via iSCSI to another Solaris >> box which used them to form another zpool and had WCE turned on would >> it be reliable? >> > > Nope. That's because all the iSCSI are in the same fault > domain as they share a unified back-end cache. What works, > in principle, is mirroring SCSI channels hosted on > different storage controllers (or N SCSI channels on N > controller in a raid group). > > Which is why keeping the WC set to the default, is really > better in general.
Well I was actually talking about two backend Solaris storage servers serving up storage over iSCSI to a front-end Solaris server serving ZFS over NFS, so I have redundancy there, but want the storage to be performant, so I want the iSCSI to have WCE, yet I want it to be reliable and have it honor cache flush requests from the front-end NFS server. Does that make sense? Is it possible? -Ross _______________________________________________ zfs-discuss mailing list zfs-discuss@opensolaris.org http://mail.opensolaris.org/mailman/listinfo/zfs-discuss
