IDS Forum

Re: Esoteric Disk Issues

Posted By: Art Kagel
Date: Friday, 3 April 2009, at 4:21 p.m.

In Response To: RE: Esoteric Disk Issues (Norma Jean Sebastian)

What's wrong with ZFS for databases is that it was designed for filesystems.

From the OpenSolaris ZFS pages (
http://www.opensolaris.org/os/community/zfs/whatis/):

ZFS presents a pooled storage model that completely eliminates the concept
of volumes and the associated problems of partitions, provisioning, wasted
bandwidth and stranded storage. Thousands of file systems can draw from a
common storage pool, each one consuming only as much space as it actually
needs. The combined I/O bandwidth of all devices in the pool is available to
all filesystems at all times.

All operations are copy-on-write transactions, so the on-disk state is
always valid. There is no need to fsck(1M) a ZFS file system, ever. Every
block is checksummed to prevent silent data corruption, and the data is
self-healing in replicated (mirrored or RAID) configurations. If one copy is
damaged, ZFS detects it and uses another copy to repair it.

ZFS introduces a new data replication model called RAID-Z. It is similar to
RAID-5 but uses variable stripe width to eliminate the RAID-5 write hole
(stripe corruption due to loss of power between data and parity updates).
All RAID-Z writes are full-stripe writes. There's no read-modify-write tax,
no write hole, and — the best part — no need for NVRAM in hardware. ZFS
loves cheap disks.

But cheap disks can fail, so ZFS provides disk scrubbing. Like ECC memory
scrubbing, the idea is to read all data to detect latent errors while
they're still correctable. A scrub traverses the entire storage pool to read
every copy of every block, validate it against its 256-bit checksum, and
repair it if necessary. All this happens while the storage pool is live and
in use.
Problem #1: Note the 'shared bandwidth' . Your database chunks are
competing for bandwidth with every other ZFS filesystem on the local net.

Problem #2: Copy-on-write means that data pages are always being moved from
location to location every time they are written to. Later the ZFS has to
go back and remove the previous copy of the page. More wasted bandwidth and
IO capacity.

Problem #3: ZFS uses RAID-Z which is just a fancy RAID6 ie RAID5 but with
two parity disks. Yes it can use RAID1, but what SA is going to pass on
configuring the hot new RAIDZ? (Yes, ZFS is proactive about repairing bad
pages if the filesystem is protected by RAID5, RAIDZ, or RAID1. I do like
that part, but does it work over the long haul?)

Problem #4: ZFS loves cheap (read 'slow') disks. What sales person could
pass up on selling you the cheapest and hence the slowest disks around?

Problem #5: Every block read and every block write has to use up CPU
bandwidth calculating per block checksums.

Do I really have to go on?

Art S. Kagel
Oninit (www.oninit.com)
IIUG Board of Directors (art@iiug.org)

Disclaimer: Please keep in mind that my own opinions are my own opinions and
do not reflect on my employer, Oninit, the IIUG, nor any other organization
with which I am associated either explicitly or implicitly. Neither do
those opinions reflect those of other individuals affiliated with any entity
with which I am affiliated nor those of the entities themselves.

On Fri, Apr 3, 2009 at 3:07 PM, Norma Jean Sebastian <
nsebastian@flinnsci.com> wrote:

> Hi Mike,
> You initially had said not to use ZFS with any database (or rather, you
> didn't list a specific database so 'all' might be implied)...
> is it just Informix that has a problem with ZFS?
>
> If you have links to articles about it being problematic with all
> databases, please post/send the links.
> Thanks,
> NJ
>
> -----Original Message-----
> From: ids-bounces@iiug.org [mailto:ids-bounces@iiug.org] On Behalf Of
> MIKE MAGIE
> Sent: Friday, April 03, 2009 10:36 AM
> To: ids@iiug.org
> Subject: Re: Esoteric Disk Issues [15413]
>
> Calm heads prevailed and we ended up scrapping ZFS and going with the
> good old
> Sun Volume Manager with beautifully raw devices...
>
> Thanks for all of the input - if any of you are considering using ZFS
> with
> Informix - rethink it...
>
> MM
>
> ************************************************************************
> *******
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>
>
>
> *******************************************************************************
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>

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Messages In This Thread

• Esoteric Disk Issues
  MIKE MAGIE -- Tuesday, 31 March 2009, at 11:18 a.m.
  • Re: Esoteric Disk Issues
    rroussey@comcast.net -- Tuesday, 31 March 2009, at 1:30 p.m.
  • RE: Esoteric Disk Issues
    Norma Jean Sebastian -- Tuesday, 31 March 2009, at 1:39 p.m.
    • Re: RE: Esoteric Disk Issues
      MIKE MAGIE -- Tuesday, 31 March 2009, at 3:31 p.m.
      • RE: RE: Esoteric Disk Issues
        Norma Jean Sebastian -- Wednesday, 1 April 2009, at 10:16 a.m.
        • RE: RE: Esoteric Disk Issues
          Norma Jean Sebastian -- Wednesday, 1 April 2009, at 10:44 a.m.
  • Re: Esoteric Disk Issues
    Obnoxio The Clown -- Thursday, 2 April 2009, at 11:12 a.m.
    • Re: Esoteric Disk Issues
      Art Kagel -- Thursday, 2 April 2009, at 3:39 p.m.
      • Re: Esoteric Disk Issues
        MIKE MAGIE -- Friday, 3 April 2009, at 11:36 a.m.
        • RE: Esoteric Disk Issues
          Norma Jean Sebastian -- Friday, 3 April 2009, at 3:07 p.m.
          • RE: Esoteric Disk Issues
            Norma Jean Sebastian -- Friday, 3 April 2009, at 4:05 p.m.
            • Re: Esoteric Disk Issues
              Art Kagel -- Friday, 3 April 2009, at 4:28 p.m.
          • Re: Esoteric Disk Issues
            Art Kagel -- Friday, 3 April 2009, at 4:21 p.m.

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