zfs discuss - Feb 2009 - A question on "non-consecutive disk failures"

>From the presentation "ZFS - The last word in filesystems", Page
22
	"In a multi-disk pool, ZFS survives any non-consecutive disk
failures"

Questions:
If I have a 3 disk RAIDZ with disks A, B and C, then:
- if disk b fails, then will I be able to continue to read data if
disks A and C are still available ?
If I have a 4 disk RAIDZ with disks A, B, C, and D, then:
- if disks a and b fail, then I won''t be able to read from the mirror
any more. Is this understanding correct ?
- if disks a and c fail, then I will be be able to read from disks b
and d. Is this understanding correct ?

-- Sriram

On February 8, 2009 12:04:22 AM +0530 Sriram Narayanan <sriram at
belenix.org>
wrote:
>> From the presentation "ZFS - The last word in filesystems",
Page 22
> 	"In a multi-disk pool, ZFS survives any non-consecutive disk
failures"
>
> Questions:
> If I have a 3 disk RAIDZ with disks A, B and C, then:
> - if disk b fails, then will I be able to continue to read data if
> disks A and C are still available ?
yes.  raidz allows for 1 disk failure.
> If I have a 4 disk RAIDZ with disks A, B, C, and D, then:
> - if disks a and b fail, then I won''t be able to read from the
mirror
> any more. Is this understanding correct ?
what mirror?  there is no mirror.  you have a raidz.  you can have 1
disk failure.
> - if disks a and c fail, then I will be be able to read from disks b
> and d. Is this understanding correct ?
no.  you will lose all the data if 2 disks fail.

The part of the slides you are referring to is in reference to ditto
blocks, which allow failure of PARTS of a SINGLE disk.

-frank

On Sat, Feb 7, 2009 at 6:34 PM, Sriram Narayanan <sriram at belenix.org>
wrote:
> >From the presentation "ZFS - The last word in filesystems",
Page 22
>        "In a multi-disk pool, ZFS survives any non-consecutive disk
failures"
>
> Questions:
> If I have a 3 disk RAIDZ with disks A, B and C, then:
> - if disk b fails, then will I be able to continue to read data if
> disks A and C are still available ?
> If I have a 4 disk RAIDZ with disks A, B, C, and D, then:
> - if disks a and b fail, then I won''t be able to read from the
mirror
> any more. Is this understanding correct ?
> - if disks a and c fail, then I will be be able to read from disks b
> and d. Is this understanding correct ?
No. That quote is part of the discussion of ditto blocks.

See the following:

http://blogs.sun.com/bill/entry/ditto_blocks_the_amazing_tape

-- 
-Peter Tribble
http://www.petertribble.co.uk/ - http://ptribble.blogspot.com/

On Sun, Feb 8, 2009 at 1:56 AM, Peter Tribble <peter.tribble at gmail.com>
wrote:
<snip/>
> No. That quote is part of the discussion of ditto blocks.
>
> See the following:
>
> http://blogs.sun.com/bill/entry/ditto_blocks_the_amazing_tape
>
Thank you, Peter.

-- Sriram

On Sun, Feb 8, 2009 at 1:29 AM, Frank Cusack <fcusack at fcusack.com>
wrote:
>
> what mirror?  there is no mirror.  you have a raidz.  you can have 1
> disk failure.
Thanks for the correction. I was thinking RAIDZ, but typed "mirror". I
have only RAIDZs on my servers.
>
>> - if disks a and c fail, then I will be be able to read from disks b
>> and d. Is this understanding correct ?
>
> no.  you will lose all the data if 2 disks fail.
>
> The part of the slides you are referring to is in reference to ditto
> blocks, which allow failure of PARTS of a SINGLE disk.
>
Thanks. I''ve started to read the various ZFS documentation.


-- Sriram

Sriram,

On Sun, Feb 08, 2009 at 12:04:22AM +0530, Sriram Narayanan
wrote:
> >From the presentation "ZFS - The last word in filesystems",
Page 22
> 	"In a multi-disk pool, ZFS survives any non-consecutive disk
failures"
> 
> Questions:
> If I have a 3 disk RAIDZ with disks A, B and C, then:
> - if disk b fails, then will I be able to continue to read data if
> disks A and C are still available ?
Yes, with RAIDZ, ZFS can sustain with 1 disk failure.
> If I have a 4 disk RAIDZ with disks A, B, C, and D, then:
> - if disks a and b fail, then I won''t be able to read from the
mirror
> any more. Is this understanding correct ?
Since, this is a RAIDZ configuration, it can sustain upto one disk failure.
So, answer is "no".

RAIDZ is similar to RAID-5 wrt to redundancy.

> - if disks a and c fail, then I will be be able to read from disks b
> and d. Is this understanding correct ?
Same as above.

However, if you had configured it as RAIDZ-2 then it can sustain upto 2 disk
failures.


The other option would be configure it as mirrored pool. And this too can
sustain 2 disk failures (one in each mirror device).

Hope that helps.

Thanks and regards,
Sanjeev
> 
> -- Sriram
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