best practices using LVM and e2fsck

[Edward Ned Harvey]

> From: Derek Martin [mailto:invalid-yPs96gJSFQo51KKgMmcfiw at public.gmane.org]
> 
> On Mon, Jun 28, 2010 at 09:35:27PM -0400, Edward Ned Harvey wrote:
> > I do all of this on solaris/opensolaris with ZFS.  The ability to
> > scrub a filesystem while it's mounted and in use seems like such a
> brainless
> > obvious feature.
> 
> Not to me, it doesn't...  From what I've read, it generates a lot of
> I/O (as one would expect), and if so I doubt I'd ever want to be using
> a filesystem where a scrub is going on.  I'd put this functionality in

Allow me to rephrase:
In ext3/4, you have no choice.  You need to fsck occasionally, and you must
do it while the filesystem is dismounted.  Often, you don't even have a
choice about *which* dismount will cause it to fsck.

In ZFS, you have a choice.  You can scrub while the system is not in use if
you want, or you can scrub while it's in use, and acknowledge that
performance will be lower than usual during that time.  If you wanted, you
could simply never scrub.  You have that option.


> It also is prone to fragmentation, which in the long run may degrade
> performance, and it has no defrag utility.

If you keep snapshots around, this is true.  If you don't do snapshots, it's
not true.

Fragmentation is inherently a characteristic that goes hand-in-hand with
copy-on-write, which is the key technology that enables snapshots.  Most
people who go to ZFS are doing it because we're really really happy to have
snapshots, and we're perfectly happy to pay for it in terms of
fragmentation.  I am sure you can measure and detect the fragmentation if
you want.  But neither I, nor any of my users have ever noticed it.

BTW, snapshots & copy-on-write are the key technologies that enable instant
block-level diff incremental backups.  ;-)  Thanks to this, my nightly
backups which formerly required 10 hrs per night for rsync to scan the tree
for changed items ... Now require an average 7 mins per night, because no
scan is required to search for things that changed.  Given this, versus
fragmentation which we haven't even noticed, *plus* the option of scrubbing
online if you want to, and the ability to restore things from snapshot
instead of needing the backup media ...  Means I am very *thoroughly* happy
we made this move to ZFS.

One more thing.  Unrelated except that it goes along with the "compare ZFS
to ExtN" subject:
Because ZFS handles the posix (filesystem) layer, integrated with the
block-level (raid) layer, it's able to gain performance that would simply be
impossible with traditional raid.  With traditional raid, even if you have
hardware NVRAM BBU HBA acceleration etc, if the system issues a lot of
random small writes, the disks will have to seek each of those sectors.  But
since ZFS has intimate knowledge of all of these layers, it's able to take a
bunch of small random write requests, aggregate them, and choose to write
them on sequential physical blocks.  In my benchmarks, this meant a 30x
improvement in performance.


> I haven't used ZFS.  It seems pretty nice, but it's apparently not
> without its own set of limitations.  Everything in life comes with
> tradeoffs.

Always true.   ;-)