zfs discuss - Dec 2011 - 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

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: zfs-discuss-bounces at opensolaris.org
[mailto:zfs-discuss-bounces at opensolaris.org] On Behalf Of Thomas Nau
Sent: Sunday, December 11, 2011 1:28 PM
To: zfs-discuss at opensolaris.org
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
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zfs-discuss at opensolaris.org
http://mail.opensolaris.org/mailman/listinfo/zfs-discuss

Corruption?  Or just loss?

On Sun, Dec 11, 2011 at 1:27 PM, Matt Breitbach
<matthewb at flash.shanje.com>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: zfs-discuss-bounces at opensolaris.org
> [mailto:zfs-discuss-bounces at opensolaris.org] On Behalf Of Thomas Nau
> Sent: Sunday, December 11, 2011 1:28 PM
> To: zfs-discuss at opensolaris.org
> 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
> zfs-discuss at opensolaris.org
> http://mail.opensolaris.org/mailman/listinfo/zfs-discuss
>
>
> _______________________________________________
> zfs-discuss mailing list
> zfs-discuss at opensolaris.org
> http://mail.opensolaris.org/mailman/listinfo/zfs-discuss
>
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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:Garrett.DAmore at nexenta.com] 
Sent: Sunday, December 11, 2011 10:35 PM
To: Frank Cusack
Cc: Matt Breitbach; zfs-discuss at opensolaris.org
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" <frank at
linetwo.net> wrote:

Corruption?  Or just loss?

On Sun, Dec 11, 2011 at 1:27 PM, Matt Breitbach <matthewb at
flash.shanje.com>
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: zfs-discuss-bounces at opensolaris.org
[mailto:zfs-discuss-bounces at opensolaris.org] On Behalf Of Thomas Nau
Sent: Sunday, December 11, 2011 1:28 PM
To: zfs-discuss at opensolaris.org
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
zfs-discuss at opensolaris.org
http://mail.opensolaris.org/mailman/listinfo/zfs-discuss


_______________________________________________
zfs-discuss mailing list
zfs-discuss at opensolaris.org
http://mail.opensolaris.org/mailman/listinfo/zfs-discuss

 

_______________________________________________
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zfs-discuss at opensolaris.org
http://mail.opensolaris.org/mailman/listinfo/zfs-discuss

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> From: zfs-discuss-bounces at opensolaris.org [mailto:zfs-discuss-
> bounces at opensolaris.org] 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.