On Wed, 2006-07-19 at 11:53 +0100, Peter Grandi wrote:
> [ ... ]
> mingz> when u say large parallel storage system, you mean
> mingz> independent spindles right? but most people will have all
> mingz> disks configured in one RAID5/6 and thus it is not parallel
> mingz> any more.
> cw> it depends, you might have 100s of spindles in groups, you
> cw> don't make a giant raid5/6 array with that many disks, you
> cw> make a number of smaller arrays
> Perhaps you are undestimating the ''if it can be done''
> Also, if one does a number of smaller RAID5s, is each one a
> separate filesystem or they get aggregated, for example with
> LVM with ''concat''? Either way, how likely is is that the
> consequences have been thought through?
> I would personally hesitate to recommend either, especially a
> two-level arrangement where the base level is a RAID5.
could u give us some hints on this? since it is really popular to have a
FS/LV/MD structure and I believe LVM is designed for this purpose.
> [I am making an effort in this discussion to use euphemisms]
> mingz> i think with write barrier support, system without UPS
> mingz> should be ok.
> cw> with barrier support a UPS shouldn't be necessary
> Sure, «should» and «shouldn't» are nice hopeful concepts.
> But write barriers are difficult to achieve, and when achieved
> they are often unreliable, except on enterprise level hardware,
> because many disks/host adapters/... simply lie as to whether
> they have actually started writing (never mind finished writing,
> or written correctly) stuff.
> To get reliable write barrier often one has to source special
> cards or disks with custom firmware; or leave system integration
> to the big expensive guys and buy an Altix or equivalent system
> from Sun or IBM.
> Besides I have seen many reports of ''corruption'' that cannot
> be fixed by write barriers: many have the expectation that
> *data* should not be lost, even if no 'fsync' is done, *as if*
> 'mount -o sync' or 'mount -o data=ordered'.
> Of course that is a bit of an inflated expectation, but all that
> the vast majority of sysadms care about is whether it ''just
> works'', without ''wasting time'' figuring things out.
> mingz> considering even u have UPS, kernel oops in other parts
> mingz> still can take the FS down.
> cw> but a crash won't cause writes to be 'reordered' [ ... ]
> The metadata will be consistent, but metadata and data may well
> will be lost. So the filesystem is still ''corrupted'', at least
> from the point of view of a sysadm who just wants the filesystem
> to be effortlessly foolproof. Anyhow, if a crash happens all
> bets are off, because who knows *what* gets written.
> Look at it from the point of view of a ''practitioner'' sysadm:
> ''who cares if the metadata is consistent, if my 3TiB
> application database is unusable (and I don't do backups
> because after all it is a concat of RAID5s, backups are not
> necessary) as there is a huge gap in some data file, and my
> users are yelling at me, and it is not my fault''
> The tradeoff in XFS is that if you know exactly what you are
> doing you get extra performance...
then i think unless you disable all write cache, none of the file system
can achieve this goal. or maybe ext3 with both data and metadata into
log might do this?