[OpenIndiana-discuss] Zfs stability "Scrubs"

[Michael Stapleton]

Some basic thoughts:


The one advantage of using a storage array instead of a JBOD is the
write cache when doing random writes. But the cost is that you loose the
data integrity features if the ZFS pool is not configured with
redundancy.

ZFS works best when it has multiple direct paths to multiple physical
devices configured with mirrored VDevs.

So the bottom line for ZFS is that JBODs are almost always the best
choice as long as the quality of the devices and device drivers are
similar.

SANs provide centralized administration and maintenance, which is their
main feature. 

If you could map actual hard drives from the SAN to ZFS everyone could
be happy.

Backup done while services are running all too often results in unhappy
people.

There are few easy answers when it comes for performance.

And the actual answer to most questions is "It Depends".


Mike




On Sat, 2012-10-13 at 17:02 +0400, Jim Klimov wrote:

> 2012-10-13 7:26, Michael Stapleton wrote:
> > The VAST majority of data centers are not storing data in storage that
> > does checksums to verify data, that is just the reality. Regular backups
> > and site replication rule.
> 
> And this actually concerns me... we help maintain some deployments
> built by customers including professional arrays like Sun Storagetek
> 6140 serving a few LUNs to directly attached servers (so it happens).
> 
> The arrays are black boxes to us - we don't know if they use
> something block-checksummed similar to ZFS inside, or can only
> protect against whole-disk failures, when a device just stops
> responding?
> 
> We still have little idea - in what config would the data be
> safer to hold a ZFS pool, and which should give more performance:
> * if we use the array with its internal RAID6, and the client
>    computer makes a pool over the single LUN
> * a couple of RAID6 array boxes in a mirror provided by arrays'
>    firmware (independently of client computers, who see a MPxIO
>    target LUN), and the computer makes a pool over the single
>    multi-pathed LUN
> * a couple of RAID6 array boxes in a mirror provided by ZFS
>    (two independent LUNs mirrored by computer)
> * serve LUNs from each disk in JBOD manner from the one or two
>    arrays, and have ZFS construct pools over that.
> 
> Having expensive hardware RAIDs (anyway available on customer's
> site) serving as JBODs is kind of overkill - any well-built JBOD
> costing a fraction of this array could suffice. But regarding
> data integrity known to be provided by ZFS and unknown to be
> really provided by black-box appliances, downgrading the arrays
> to JBODs might be better. Who knows?.. (We don't, advice welcome).
> 
> 
> 
> There are several more things to think about:
> 
> 1) Redundant configs without knowledge of which side of the mirror
>     is good, or what permutation of RAID blocks yields the correct
>     answer, is basically useless, and it can propagate errors by
>     overwriting an unknownly-good copy of the data with unknownly-
>     corrupted one.
> 
>     For example, take a root mirror. You find that your OS can't
>     boot. You can try to split the mirror into two separate disks,
>     fsck each of them and if one is still correct, recreate the
>     mirror using it as base (first half). Even if both disks give
>     some errors, these might be in different parts of the data, so
>     you have a chance of reconstructing the data using these two
>     halves and/or backups. However, if your simplistic RAID just
>     copies data from disk1 to disk2 in case of any discrepancies
>     and unclean shutdowns, you're roughly 50% likely to corrupt a
>     good disk2 with bad data from disk1.
> 
>     This setup assumed that bit-rot never occurred or was too rare,
>     bus/RAM errors never happened or were ruled out by CRC/ECC,
>     and instead disks died altogether, instantly becoming bricks
>     (which could be quite true in the old days, and can still be
>     probable with expensive enterprise hardware). Basically, this
>     assumed that data written from a process was the same data that
>     hit the disk platters and the same data that was returned upon
>     reads (unless an IO error/deviceMissing were reported) - in that
>     case old RAIDs could indeed propagate assumed-good data onto
>     replacement disk(s) during reconstruction of the array.
> 
> 2) Backups and replicas without means to verify them (checksums
>     or at least three-way comparisons at some level) are also
>     tainted, because you don't really know if what you read from
>     them ever matches what you wrote to them (perhaps several years
>     ago, counting from the moment the data was written onto RAID
>     originally).
> 
> My few cents,
> //Jim