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We use RAID1+0 with md on Linux (currently 2.6.37) to create an md device, then use LVM to provide volume management on top of the device, and then use ext4 as our filesystem on the LVM volume groups.

With SSDs as the drives, we'd like to see the TRIM commands propagate through the layers (ext4 -> LVM -> md -> SSD) to the devices.

It looks like recent 2.6.3x kernels have had a lot of new SSD-related TRIM support added, including lots more coverage of Device Mapper scenarios, but we still can't seem to get it to cascade down properly.

Is this possible yet? If so, how? If not, is any progress being made?

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See updated serverfault.com/a/229486/67675 :) – poige Apr 7 '12 at 3:30
and serverfault.com/questions/227918/… – poige Jul 11 '12 at 1:13

6 Answers

up vote 6 down vote accepted

Not possible, cause Linux Soft RAID doesn't support this (yet?).

UPD.: Thanks to Wodin for letting me know — according to lkml.org/lkml/2012/3/11/261 this functional has been addedproposed recently. proposed != accepted, though.

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You can still minus this answer, but this wouldn't make LSR support TRIM :) – poige Feb 2 '11 at 2:26
1  
It does now. lkml.org/lkml/2012/3/11/261 – Wodin Apr 6 '12 at 23:29
@Wodin, thanks. – poige Apr 7 '12 at 3:24
@Wodin, I can't find the proposed changes even in 3.5. They don't seem to be accepted. – poige Jul 11 '12 at 1:10
up vote 11 down vote
+50

As of 2.6.37, it should be present (source). The kernel doesn't do it in the background, the block discard process is currently designed to be run on demand (cron script!). Dm-crypt support doesn't exist yet.

On January 13th, 2011 a patch was merged into dm-raid1.c that reads: 

dm raid1: support discard

Enable discard support in the DM mirror target.
Also change an existing use of 'bvec' to 'addr' in the union.

I'm not 100%, but I think that's the merge-window for 2.6.38.

EXT4 added support a while ago, as did LVM. RAID is the one key that doesn't have support. As of 1/13/2011, it appears support has been added. Look for it in 2.6.38 or maybe 2.6.39.

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Saw that thread, and the related commits, but like I said in the question - does this mean it'll properly pass through not only LVM (Device Mapper) but also md (Software RAID)? – Don MacAskill Jan 30 '11 at 7:01
1  
This sounds promising, but my understanding is that dmraid is primarily used for so-called 'fakeRAID' hardware RAID controllers. What I'm talking about is a more typical software-only mdadm RAID-1 (+0) array(s) with LVM on top. AFAIK, that setup doesn't benefit from dmraid's new-found ability to handle discards. Right? – Don MacAskill Feb 1 '11 at 21:59
1  
@DonMacAskill The 'raid1.c' file doesn't have any commits referencing TRIM, FITRIM, or 'dispose'. So, looks like mdadm --create RAID support isn't there yet after all. – sysadmin1138♦ Feb 1 '11 at 22:25
@sysadmin1138 I think you mean discard, which is what it's called within the Linux kernel. And that patch does seem to refer to discard. – Michael Hampton Aug 30 '12 at 20:10
up vote 1 down vote

Mdtrim may need more working:

Cyberax-mdtrim-0a40e8d# ./mdtrim.py -m /dev/md4 -s /home
Scratch directory is /home, trimmer file size is 0 GB 790 MB
Found slave sdc2 on /dev/sdc with MD offset 0 and partition offset 249856
Creating trimmer file
252,2: device not found in /dev
Traceback (most recent call last):
  File "./mdtrim.py", line 120, in <module>
    if lines[2].find("assuming %d byte sectors" % sector_size) == -1:
IndexError: list index out of range
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up vote 0 down vote

You can use my MDTRIM script ( https://github.com/Cyberax/mdtrim/ ) to TRIM empty space on ext4/3 level-1 RAIDs. We start it periodically from cron and it works great for us.

Adding support for other RAID levels is possible, but I don't have time (or need) for that.

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up vote 0 down vote

As suggested here You can use

lsblk -D

in order to check if your blockdevices pass through the discard commands.

Also note that the section devices in lvm.conf contains an option issue_discards. See

man 5 lvm.conf

for more info.

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up vote -1 down vote

Does this technique do what you want:

http://www.ocztechnologyforum.com/forum/showthread.php?82648-software-RAID-LVM-TRIM-support-on-Linux

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1  
While this link may answer the question, it is better to include the essential parts of the answer here and provide the link for reference. Link-only answers can become invalid if the linked page changes. – Michael Hampton Nov 26 '12 at 22:11

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