On Mar 17, 2008, at 4:14 PM, Daniel Bye wrote:
On Mon, Mar 17, 2008 at 07:34:04PM +0000, Pollywog wrote:
On Monday 17 March 2008 19:17:58 Wojciech Puchar wrote:
i would say it's absolutely needed.
anyway - any reason to not use soft updates on every filesystem?
What exactly is a soft update?
It's a bit like a hard update, but it won't hurt your disks as much
if your system crashes... ;-P
On a more serious note, it's a technique for ensuring the integrity of
disks after a system crash or power failure. Like journalling, they
don't guarantee data won't be lost, but instead that the disks will be
in a consistent state at recovery.
Soft updates is a means of re-ordering the writes to a filesystem such
that the complete filesystem, both data and meta data, remains
reasonably consistent during the writing process. This consistency is
necessary insurance in case of a system crash or power failure during
the writing process. Soft updates seeks to re-order the writes in such
a way that the filesystem can be safely recovered by an automatic fsck
process when the system is restarted. At the same time soft updates
works to maintain high system performance . Previous to soft updates
you could either mount the filesystem synchronously or asynchronously.
With Synchronous mounts the filesystem meta data writes were handled
before data writes. This caused excessive and expensive seeking from
the disk mechanism as it moved from one part of the disk to update the
meta-data to the other part of the disk to write the application data.
With an asynchronous mount the kernel was free to perform the writes
in the order most beneficial for performance but if the system crashed
in the middle of a write one could expect a very difficult situation
for fsck to fix.
My squid is on OpenBSD. My cache partition is spread across two
spindles of a drive provided by the ccd driver mounted either
asynchronously or with soft updates. Either way is fine because if my
squid machine were to crash so hard that the cache partition was toast
it wouldn't take but 10 minutes rebuild the filesystem from scratch
and use squid -z to reinitialize it. For me there's really no data on
there worthy of softupdates.
-- Chris
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