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Re: [Qemu-discuss] Virtio-blk and trim

From: Brian Rak
Subject: Re: [Qemu-discuss] Virtio-blk and trim
Date: Wed, 01 Oct 2014 18:18:57 -0400
User-agent: Mozilla/5.0 (Windows NT 6.1; WOW64; rv:24.0) Gecko/20100101 Thunderbird/24.6.0

It does make a lot of sense, if you're using LVM as a backend for the guest. LVM handles discard commands by zeroing out the relevant data. This means that if you take a backup of the guest from the host, you aren't backing up all the old, unused data.

Running something like fstrim from the host will not do this.

On 10/1/2014 4:24 PM, Tony Su wrote:
  I don't know that it makes any sense to run a TRIM/discard function
in a virtual file system.
It only makes sense for the command to be applied directly to the disk
at the hardware I/O level.

So, if you run the command on your HostOS regularly, it should addres
the needs for all GuestOS as well as the HostOS.


On Wed, Oct 1, 2014 at 1:19 PM, George Shuklin <address@hidden> wrote:
It's better to name it 'discard', because TRIM a very specific ATA command.
It done completely different in SCSI and slightly different in SD cards. But
linux call it 'discard', or, to be precise, BLKDISCARD (name of ioctl).

On 10/01/2014 02:21 AM, Brian Rak wrote:
Are there any plans to bring trim support to Virtio-blk?  We've got a
couple things that would be made nicer by having working trim support.

I see that it's implemented in virtio-scsi, but there's a decent
performance hit for switching over to that.

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