Re: [zfs-discuss] zfs_nocacheflush, nvram, and root pools
On 12/02/08 03:47, River Tarnell wrote: hi, i have a system connected to an external DAS (SCSI) array, using ZFS. the array has an nvram write cache, but it honours SCSI cache flush commands by flushing the nvram to disk. the array has no way to disable this behaviour. a well-known behaviour of ZFS is that it often issues cache flush commands to storage in order to ensure data integrity; while this is important with normal disks, it's useless for nvram write caches, and it effectively disables the cache. so far, i've worked around this by setting zfs_nocacheflush, as described at [1], which works fine. but now i want to upgrade this system to Solaris 10 Update 6, and use a ZFS root pool on its internal SCSI disks (previously, the root was UFS). the problem is that zfS_nocacheflush applies to all pools, which will include the root pool. my understanding of ZFS is that when run on a root pool, which uses slices (instead of whole disks), ZFS won't enable the write cache itself. i also didn't enable the write cache manually. so, it _should_ be safe to use zfs_nocacheflush, because there is no caching on the root pool. am i right, or could i encounter problems here? Yes you are right and this should work. You may want to check that the write cache is disabled on the root pool disks using 'format -e' + cache + write_cache + display. (the system is an NFS server, which means lots of synchronous writes (and therefore ZFS cache flushes), so i *really* want the performance benefit from using the nvram write cache.) Indeed, performance would be bad without it. - river. Neil. ___ zfs-discuss mailing list zfs-discuss@opensolaris.org http://mail.opensolaris.org/mailman/listinfo/zfs-discuss
[zfs-discuss] zfs_nocacheflush, nvram, and root pools
-BEGIN PGP SIGNED MESSAGE- Hash: SHA1 hi, i have a system connected to an external DAS (SCSI) array, using ZFS. the array has an nvram write cache, but it honours SCSI cache flush commands by flushing the nvram to disk. the array has no way to disable this behaviour. a well-known behaviour of ZFS is that it often issues cache flush commands to storage in order to ensure data integrity; while this is important with normal disks, it's useless for nvram write caches, and it effectively disables the cache. so far, i've worked around this by setting zfs_nocacheflush, as described at [1], which works fine. but now i want to upgrade this system to Solaris 10 Update 6, and use a ZFS root pool on its internal SCSI disks (previously, the root was UFS). the problem is that zfS_nocacheflush applies to all pools, which will include the root pool. my understanding of ZFS is that when run on a root pool, which uses slices (instead of whole disks), ZFS won't enable the write cache itself. i also didn't enable the write cache manually. so, it _should_ be safe to use zfs_nocacheflush, because there is no caching on the root pool. am i right, or could i encounter problems here? (the system is an NFS server, which means lots of synchronous writes (and therefore ZFS cache flushes), so i *really* want the performance benefit from using the nvram write cache.) - river. [1] http://www.solarisinternals.com/wiki/index.php/ZFS_Evil_Tuning_Guide#Cache_Flushes -BEGIN PGP SIGNATURE- iD8DBQFJNRJVIXd7fCuc5vIRAgDlAJ0boVf5zmvkRySeIHVumsKm3VSVhACffyOK POEMyzG8U2yQYeZr01uJ74Q= =9eBp -END PGP SIGNATURE- ___ zfs-discuss mailing list zfs-discuss@opensolaris.org http://mail.opensolaris.org/mailman/listinfo/zfs-discuss
Re: [zfs-discuss] zfs_nocacheflush
Peter Tribble writes: A question regarding zfs_nocacheflush: The Evil Tuning Guide says to only enable this if every device is protected by NVRAM. However, is it safe to enable zfs_nocacheflush when I also have local drives (the internal system drives) using ZFS, in particular if the write cache is disabled on those drives? What I have is a local zfs pool from the free space on the internal drives, so I'm only using a partition and the drive's write cache should be off, so my theory here is that zfs_nocacheflush shouldn't have any effect because there's no drive cache in use... Seems plausible, But I'd check that the caches are indeed off using format -e. -r -- -Peter Tribble http://www.petertribble.co.uk/ - http://ptribble.blogspot.com/ ___ zfs-discuss mailing list zfs-discuss@opensolaris.org http://mail.opensolaris.org/mailman/listinfo/zfs-discuss ___ zfs-discuss mailing list zfs-discuss@opensolaris.org http://mail.opensolaris.org/mailman/listinfo/zfs-discuss
[zfs-discuss] zfs_nocacheflush
A question regarding zfs_nocacheflush: The Evil Tuning Guide says to only enable this if every device is protected by NVRAM. However, is it safe to enable zfs_nocacheflush when I also have local drives (the internal system drives) using ZFS, in particular if the write cache is disabled on those drives? What I have is a local zfs pool from the free space on the internal drives, so I'm only using a partition and the drive's write cache should be off, so my theory here is that zfs_nocacheflush shouldn't have any effect because there's no drive cache in use... -- -Peter Tribble http://www.petertribble.co.uk/ - http://ptribble.blogspot.com/ ___ zfs-discuss mailing list zfs-discuss@opensolaris.org http://mail.opensolaris.org/mailman/listinfo/zfs-discuss
Re: [zfs-discuss] zfs_nocacheflush
Hello Peter, Wednesday, July 30, 2008, 9:19:30 AM, you wrote: PT A question regarding zfs_nocacheflush: PT The Evil Tuning Guide says to only enable this if every device is PT protected by NVRAM. PT However, is it safe to enable zfs_nocacheflush when I also have PT local drives (the internal system drives) using ZFS, in particular if PT the write cache is disabled on those drives? PT What I have is a local zfs pool from the free space on the internal PT drives, so I'm only using a partition and the drive's write cache PT should be off, so my theory here is that zfs_nocacheflush shouldn't PT have any effect because there's no drive cache in use... Should be fine in that case. -- Best regards, Robert Milkowskimailto:[EMAIL PROTECTED] http://milek.blogspot.com ___ zfs-discuss mailing list zfs-discuss@opensolaris.org http://mail.opensolaris.org/mailman/listinfo/zfs-discuss
