Re: [zfs-discuss] does log device (ZIL) require a mirror setup?
> From: [email protected] [mailto:zfs-discuss- > [email protected]] On Behalf Of Thomas Nau > > We use a STEC ZeusRAM as a log device for a 200TB RAID-Z2 pool. > As they are supposed to be read only after a crash or when booting and > those nice things are pretty expensive I'm wondering if mirroring > the log devices is a "must / highly recommended" Assuming you're running a recent version of zfs (zpool > 20 or so)... The decision to mirror or not to mirror the log device hinges around one single solitary failure condition... In normal operation, a log device is write only. Never gets read until after an ungraceful system crash. Unfortunately, it is sometimes possible for flash memory to enter a failure state which is undetected by writes, and only detected upon reads. In this state, you effectively have no log device, but you think you do. If you're in that failure state and you have an ungraceful crash, then you lose whatever you thought you had in the log. Maybe it will help if you periodically remove the log device, and then read & write the whole log device to verify it's operational (be sure to actually detect failures if any) and then re-add the log device to the pool. Seems logical. Probably nobody's ever tested it. If you have a failed unmirrored log device at the same time as an ungraceful system crash, then you lose data. (Up to 30 sec, or 5 sec worth, depending on your system.) Your decision to mirror or not to mirror all hinges around your fear of the aforementioned coincidence of log device failure & ungraceful system crash. Bear in mind, that mirroring does not eliminate the possibility of both log devices being in the same undetected failure state. It doesn't eliminate the problem, only reduces the probability. ___ zfs-discuss mailing list [email protected] http://mail.opensolaris.org/mailman/listinfo/zfs-discuss
Re: [zfs-discuss] does log device (ZIL) require a mirror setup?
Loss of bits, but depending upon the usage of the system, corruption _could_ be a possibility. I could envision an scenario where you were mapping an iSCSI lun to a system, and that system had it's own FS on top of it (think VMFS or NTFS) and when it came back online, parts of the last write commands didn't get written causing that filesystem to be corrupted. Obviously this is likely an edge case scenario, but I could see it as a possibility. The actual zpool would likely be fine and importable, but the underlying data could be corrupt if there are other filesystems layered on top of it. _ From: Garrett D'Amore [mailto:[email protected]] Sent: Sunday, December 11, 2011 10:35 PM To: Frank Cusack Cc: Matt Breitbach; [email protected] Subject: Re: [zfs-discuss] does log device (ZIL) require a mirror setup? Loss only. Sent from my iPhone On Dec 12, 2011, at 4:00 AM, "Frank Cusack" wrote: Corruption? Or just loss? On Sun, Dec 11, 2011 at 1:27 PM, Matt Breitbach wrote: I would say that it's a "highly recommended". If you have a pool that needs to be imported and it has a faulted, unmirrored log device, you risk data corruption. -Matt Breitbach -Original Message- From: [email protected] [mailto:[email protected]] On Behalf Of Thomas Nau Sent: Sunday, December 11, 2011 1:28 PM To: [email protected] Subject: [zfs-discuss] does log device (ZIL) require a mirror setup? Dear all We use a STEC ZeusRAM as a log device for a 200TB RAID-Z2 pool. As they are supposed to be read only after a crash or when booting and those nice things are pretty expensive I'm wondering if mirroring the log devices is a "must / highly recommended" Thomas ___ zfs-discuss mailing list [email protected] http://mail.opensolaris.org/mailman/listinfo/zfs-discuss ___ zfs-discuss mailing list [email protected] http://mail.opensolaris.org/mailman/listinfo/zfs-discuss ___ zfs-discuss mailing list [email protected] http://mail.opensolaris.org/mailman/listinfo/zfs-discuss ___ zfs-discuss mailing list [email protected] http://mail.opensolaris.org/mailman/listinfo/zfs-discuss
Re: [zfs-discuss] does log device (ZIL) require a mirror setup?
Corruption? Or just loss? On Sun, Dec 11, 2011 at 1:27 PM, Matt Breitbach wrote: > I would say that it's a "highly recommended". If you have a pool that > needs > to be imported and it has a faulted, unmirrored log device, you risk data > corruption. > > -Matt Breitbach > > -Original Message- > From: [email protected] > [mailto:[email protected]] On Behalf Of Thomas Nau > Sent: Sunday, December 11, 2011 1:28 PM > To: [email protected] > Subject: [zfs-discuss] does log device (ZIL) require a mirror setup? > > Dear all > We use a STEC ZeusRAM as a log device for a 200TB RAID-Z2 pool. > As they are supposed to be read only after a crash or when booting and > those nice things are pretty expensive I'm wondering if mirroring > the log devices is a "must / highly recommended" > > Thomas > ___ > zfs-discuss mailing list > [email protected] > http://mail.opensolaris.org/mailman/listinfo/zfs-discuss > > > ___ > zfs-discuss mailing list > [email protected] > http://mail.opensolaris.org/mailman/listinfo/zfs-discuss > ___ zfs-discuss mailing list [email protected] http://mail.opensolaris.org/mailman/listinfo/zfs-discuss
Re: [zfs-discuss] does log device (ZIL) require a mirror setup?
I would say that it's a "highly recommended". If you have a pool that needs to be imported and it has a faulted, unmirrored log device, you risk data corruption. -Matt Breitbach -Original Message- From: [email protected] [mailto:[email protected]] On Behalf Of Thomas Nau Sent: Sunday, December 11, 2011 1:28 PM To: [email protected] Subject: [zfs-discuss] does log device (ZIL) require a mirror setup? Dear all We use a STEC ZeusRAM as a log device for a 200TB RAID-Z2 pool. As they are supposed to be read only after a crash or when booting and those nice things are pretty expensive I'm wondering if mirroring the log devices is a "must / highly recommended" Thomas ___ zfs-discuss mailing list [email protected] http://mail.opensolaris.org/mailman/listinfo/zfs-discuss ___ zfs-discuss mailing list [email protected] http://mail.opensolaris.org/mailman/listinfo/zfs-discuss
[zfs-discuss] does log device (ZIL) require a mirror setup?
Dear all We use a STEC ZeusRAM as a log device for a 200TB RAID-Z2 pool. As they are supposed to be read only after a crash or when booting and those nice things are pretty expensive I'm wondering if mirroring the log devices is a "must / highly recommended" Thomas ___ zfs-discuss mailing list [email protected] http://mail.opensolaris.org/mailman/listinfo/zfs-discuss
