Status of SSDs under Fedora ?

John Austin ja at jaa.org.uk
Fri Jun 11 09:28:18 UTC 2010


On Thu, 2010-06-10 at 17:10 -0500, Michael Cronenworth wrote:
> Linuxguy123 wrote:
> > I'm using ext4 for that drive right now.  Is there a better filesystem
> > for SSDs ?
> 
> There's a few "SSD specific" file systems, but you won't gain much in 
> everyday computing from switching. ext4 will be just fine.
> 
> >
> > Is there anything that I should be aware of as far as switching ?
> 
> Nope! Fedora 13 has support for TRIM, so you don't need to worry about 
> anything. No tweaking, no special maintenance. Enjoy!
> 

Yes it "has TRIM support" but is it enabled and does anything use it ?

I have checked two sets of kernel documentation 

/linux-2.6.33.5/Documentation/filesystems/ext4.txt
/linux-2.6.34/Documentation/filesystems/ext4.txt

In options it says

When mounting an ext4 filesystem, the following option are accepted:
(*) == default
...
discard         Controls whether ext4 should issue discard/TRIM
nodiscard(*)            commands to the underlying block device when
                        blocks are freed.  This is useful for SSD devices
                        and sparse/thinly-provisioned LUNs, but it is off
                        by default until sufficient testing has been done.


When I mount an ext4 SSD (No RAID) in F13 I see no "discard" option set, maybe the default
has been changed but I doubt it.

Hot-plugging my 128GB Crucial SSD shows
/dev/sdb3 on /media/ssd3 type ext3 (rw,nosuid,nodev)
/dev/sdb2 on /media/ssd2_ type ext4 (rw,nosuid,nodev)
/dev/sdb1 on /media/ssd1 type ext3 (rw,nosuid,nodev)

Partition sdb2 is "/" of a full F13 ext4 install

The fstab on that ext4 F13 partition shows
UUID=ededfd60-3173-40b7-b144-026143e3cbf7  /  ext4  defaults  1 1

So the installer does not set "discard"

A manual 
mount -t auto -o discard /dev/sdb2 /mnt/zip
gives
/dev/sdb2 on /mnt/zip type ext4 (rw,discard)

I assume that setting "discard" in /etc/fstab would work
-----------------------------------------------------

It would be highly desirable to have definitive answers to

1. Is "discard" set by default in F13?

2. How can you find what/where the default mount options are set

3. Does setting the "discard" option do everything that is required to enable
TRIM to do its job with no further action?

4. Is it safe to use the "discard" option yet?

5. How do you automatically set the "discard" option when hot-plugging an SSD disk?
   (What controls the mount? hal, udev, XFCE, Gnome, kde, ...)
   I am using kdm, XFCE and probably bits of gnome

John






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