Filesystems: Linux versus FreeBSD

Wed Sep 21 10:48:59 PDT 2005

Hello,

NO I AM NOT LOOKING TO START A FLAMEWAR.  I'm a FreeBSD admin who is
soliciting wisdom from Linux admins. :)

Okay, that out of the way.

I'm looking to build a production service pod.  Two tiers - web server,
db server.  We are using PostgreSQL and FreeBSD.  No database
replication, just yet.

Our incumbent failover strategy is to have two database servers, and
dump the database every 30 minutes, copy it to the backup db server.  IF
the primary DB server fails, we turn our web application to the "down
page" and load the database on the backup DB, and resume service from
there.

Instead of building two expensive DB servers with screamin'-fast disks,
I have wandered pretty far down the path to putting a disk appliance in
the rack, that can provide high performance and high availability.  If a
database server fails, I can take that out of production and connect the
backup to our storage backend, run FS-level and pg-level consistency
checks, and get back to work.

One nice thing about FreeBSD is that the SoftUpdates feature bundles
writes intelligently to keep the filesystem in a sufficiently consistent
state, so that most of the time a fsck operation is not required to
bring the server back up even after a hard reset.

Using FreeBSD, my normal failover procedure will allow me to boot, mount
unclean filesystem, run pg consistency check, and load database, and
fsck in the background.

UNFORTUNATELY FreeBSD's support for vendor-approved HBAs tends to be,
shall we say, spotty ... there is a very good chance that we will want
to replace the database OS with Fedora Core ... which leads me to ask,
does Linux these days have an FS option that can offer similar
advantages to SoftUpdates, such that I needn't fsck after a server
crash?  Is it robust?

And to answer the obvious question, YES I understand that relying on
SoftUpdates means that it is possible for certain DB transactions to be
lost shortly before the server crash.  We have determined that this is
an acceptible risk.

Thanks for the advice. :)

Sincerely,
-danny

-- 
http://dannyman.toldme.com/