From: Brassow Jonathan <jbrassow@redhat.com>
To: oranenj+lvm@iki.fi,
LVM general discussion and development <linux-lvm@redhat.com>
Subject: Re: [linux-lvm] Does LVM RAID1 have TRIM support?
Date: Tue, 2 Sep 2014 16:49:55 -0500 [thread overview]
Message-ID: <5211F372-B9BB-43A8-B569-AC1E3B077D15@redhat.com> (raw)
In-Reply-To: <1408896497.27190.12.camel@kurenai.local>
On Aug 24, 2014, at 11:08 AM, Jarkko Oranen wrote:
> Hello
>
> Yesterday I experimented a bit with my RAID configuration on a pair of
> SSDs, and it seems that LVM's native RAID does not have TRIM support...
> At least, when I try to run fstrim manually, it complains even though
> issue_discards is enabled. Plain LVs on top of an MD RAID PV do work, of
> course.
>
> Am I perhaps missing some configuration, or do RAID1 logical volumes
> simply not have support for TRIM yet? I'm running a fairly recent kernel
> (3.15.8) and lvm version says this:
>
> LVM version: 2.02.106(2) (2014-04-10)
> Library version: 1.02.85 (2014-04-10)
> Driver version: 4.27.0
TRIM is not yet supported in LVM RAID. However, if MD has a solid TRIM implementation, it should be simple to enable it for LVM. (This is because the MD kernel modules are used to perform RAID for LVM. There is only a thin wrapper layer (linux/drivers/md/dm-raid.c) in device-mapper used to set-up the device.)
> As an aside, can anyone point me to documentation or other resources
> about the pros and cons of LVM native RAID1 setup (which I understand
> uses MD RAID internally?) vs. MD RAID PV + LVM. It seems I might be able
> to save some SSD space and only mirror the LVs I actually need to keep
> safe from crashes.
I don't know if there is a specific list to point to out there, but I can give you a couple pros/cons.
PRO:
- use one volume manager instead of two
- LVM is better suited to creating devices of varying sizes - leaving spare capacity for snapshots, etc
CONS:
- no trim support with RAID through LVM (although, I'm not sure of the state in MD)
- no reshaping (changing from one RAID type to another) capability in LVM RAID.
brassow
next prev parent reply other threads:[~2014-09-02 21:49 UTC|newest]
Thread overview: 8+ messages / expand[flat|nested] mbox.gz Atom feed top
2014-08-24 16:08 [linux-lvm] Does LVM RAID1 have TRIM support? Jarkko Oranen
2014-09-02 21:49 ` Brassow Jonathan [this message]
2014-09-04 11:06 ` Jarkko Oranen
2014-09-08 17:47 ` Brassow Jonathan
2014-09-08 18:47 ` Jarkko Oranen
2014-09-11 23:31 ` Brassow Jonathan
2014-09-12 7:24 ` Jarkko Oranen
2014-09-24 4:17 ` Brassow Jonathan
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