Re: About sparse images

[Wido den Hollander <wido@widodh.nl>]

On 08/26/2012 09:42 PM, Gregory Farnum wrote:
> On Sunday, August 26, 2012 at 11:11 AM, Sébastien Han wrote:
>> Hi all!
>>
>> I know that ceph images (rbd) are sparse. It's good but I have some
>> interrogations about the ceph -s (-w) output. Let's say your create a
>> 10G image, map it, format it and mount it. Finally you filled up the
>> entire space available. You notice that the 'data' and 'used' space
>> had grown. Now delete the content of your image but **not** this
>> image, not now. You notice that nothing changed from the ceph output,
>> it's not surprising because of the sparse capability. At this point of
>> time the output from the command ceph is neither right nor wrong. It
>> only shows the space used by the sparse image, but it doesn't mean
>> that you have 'real' data in it, just a simple object allocation. It's
>> not so relevant, the only solution is to remove the image (obviously).
>> My feeling is that we should be careful about the ceph's output, it's
>> not a good indicator for 'real' data. Ok Ceph won't replace a
>> monitoring system but I'll be happy to trust the ceph output at 100%
>> :)
>
> The output of ceph -s is intended to describe the amount of space allocated by RADOS across the OSDs. As far as any part of the Ceph project knows, the objects in those RBD images are real data and can't be removed.
> If you're interested in removing them, subsets of the RBD clients do support TRIM operations, which will remove the objects themselves. Josh or somebody will have to speak about which clients, though.
> -Greg
>

As far as I know TRIM / Discard only works with KVM and SCSI disks, 
VirtIO still seems to lack this functionality, not sure about kernel RBD.

On the filesystem side, ext4 with the "discard" mount option does this.

But it's important (like Greg already said) to note that RBD/RADOS can't 
know which object is still important and which isn't. When you unlink a 
file the filesystem doesn't zero that particular block, it just clears 
all references to it so the freed up space can be used again.

Wido

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