Re: [PATCH] fstrim: add systemd units

[Karel Zak <kzak@redhat.com>]

On Wed, Apr 09, 2014 at 02:39:54PM -0400, Theodore Ts'o wrote:
> On Wed, Apr 09, 2014 at 05:55:57PM +0200, Markus Trippelsdorf wrote:
> > > > The fixed interval is problematic. There are SSD devices out there
> > > > that suffer (their flash cells die out faster) when they get trimmed
> > > > too often. A good rule of thumb is to trim once you have written the
> > > > same amount as you have free space on your device. Obviously, that
> > > > interval varies for every user (e.g. it's one week in my case).
> > > 
> > > Is "mount -o discard" instead of fstrim interval more or less bad 
> > > regarding lifetime?
> 
> For those SSD's that have a problem, "mount -o discard" is a disaster.
> Some turn into bricks, others will have a degraded flash cells, many
> will cause extremely degraded performance for other processes.
> 
> What I usually tell people as far as who ask me for advice is that
> once a week is usually sufficient, especially for most desktop and
> server systems.  If you are running an extreme workload which is doing
> a huge number of random writes, then sure, running fstrim more
> frequently, or even using "mount -o discard" might make a lot more
> sense --- especially if you are using PCIe attached flash.  But in
> those cases, the system administrator might not want be willing to
> tolerate the random latencies in performance that might show up when
> fstrim is running (for pretty much all SATA and SAS attached SSD's out
> there, they don't yet support queued trim, so each trim command
> requires draining the NCQ queue, which is why sending trim commands,
> whether via "mount -o discard" or via fstrim will incur a performance
> penalty to whatever else might be trying to use the disk at the time).
> 
> I'll note BTW that even using "fstrim" could potentially brick an
> especially inexpensive/trashy SSD, although the vendor for whose drive
> had been most commonly accused of promulgating those to the world is
> out of business (although there are probably plenty of those SSD's
> still in use in various community distros' audiences.)

 Thanks for the advices. 
 
 I have modified the systemd fstrim.timer (daily -> weekly) and added
 some notes to the man page.

    Karel

-- 
 Karel Zak  <kzak@redhat.com>
 http://karelzak.blogspot.com