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Re: automatically running fstrim

To: Lukas Czerner <lczerner@xxxxxxxxxx>
Subject: Re: automatically running fstrim
From: Phil Karn <karn@xxxxxxxx>
Date: Wed, 25 May 2011 15:36:14 -0700
Cc: xfs@xxxxxxxxxxx
In-reply-to: <alpine.LFD.2.00.1105251335280.4667@xxxxxxxxxxxxxxxxxxxxxxxxxx>
References: <4DDBE293.8030203@xxxxxxxxxxxx> <alpine.LFD.2.00.1105251155120.4667@xxxxxxxxxxxxxxxxxxxxxxxxxx> <BANLkTi=8CX_rjWHFEjt5JO5oScdKoZopww@xxxxxxxxxxxxxx> <alpine.LFD.2.00.1105251335280.4667@xxxxxxxxxxxxxxxxxxxxxxxxxx>
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On 5/25/11 4:47 AM, Lukas Czerner wrote:

> unlinked often). Unfortunately xfs does not have this support yet, but
> other filesystems do (ext4,btrfs,...) so if you like you might try one
> of those and mount it with -o discard mount option. What it does is,
> that it will send a TRIM for every range of freed filesystem blocks.
> Generally, in its current state it comes with quite big performance
> loss (that's why we have fstrim), but in you case it might be actually
> more convenient than running fstrim all the time. Also it is handled
> automatically and the only think needed is to pass the "-i discard"
> mount option.

I have thought of using ext4 with the discard option on that device for
just this reason. But this OCZ Revo SSD seems to execute TRIM rather
slowly. I just timed it at 7 minutes 38 seconds to trim 46 GB of free
space on a 90 GB SSD. I wouldn't want that to occur in the foreground
while I'm running a program that's generating a lot of garbage blocks.

Intel drives, at least, seem to execute TRIM much faster; I think they
can take more blocks in each operation, and I conjecture that the drive
controller simply adds them to a "to do" list for later erasure in the
background. So there should probably be an option for "real-time" TRIM
on those SSDs that can do it without a performance penalty.

It would be nice if the fitrim ioctl were to issue TRIM commands only
for newly created garbage blocks that haven't already been trimmed. But
I guess that would require some major changes to the file system data
structures. At the least, it would require some special record-keeping
to keep track of this information. The Intel drive shows it's possible
to implement a very speedy TRIM, so maybe it won't be such a bad thing
to just trim the whole free list every time.


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