[ceph-users] rbd resize (shrink) taking forever and a day

Chen, Xiaoxi xiaoxi.chen at intel.com
Mon Jan 5 20:19:19 PST 2015


When you shrinking the RBD, most of the time was spent on librbd/internal.cc::trim_image(), in this function, client will iterator all unnecessary objects(no matter whether it exists) and delete them.

So in this case,  when Edwin shrinking his RBD from 650PB to 650GB,   there are[ (650PB * 1024GB/PB -650GB) * 1024MB/GB ] / 4MB/Object = 170,227,200 Objects need to be deleted.That will definitely take a long time since rbd client need to send a delete request to OSD, OSD need to find out the object context and delete(or doesn’t exist at all). The time needed to trim an image is ratio to the size needed to trim.

make another image of the correct size and copy your VM's file system to the new image, then delete the old one will  NOT help in general, just because delete the old volume will take exactly the same time as shrinking , they both need to call trim_image().

The solution in my mind may be we can provide a “—skip-triming” flag to skip the trimming. When the administrator absolutely sure there is no written have taken place in the shrinking area(that means there is no object created in these area), they can use this flag to skip the time consuming trimming.

How do you think?

From: Jake Young [mailto:jak3kaj at gmail.com]
Sent: Monday, January 5, 2015 9:45 PM
To: Chen, Xiaoxi
Cc: Edwin Peer; ceph-users at lists.ceph.com
Subject: Re: [ceph-users] rbd resize (shrink) taking forever and a day



On Sunday, January 4, 2015, Chen, Xiaoxi <xiaoxi.chen at intel.com<mailto:xiaoxi.chen at intel.com>> wrote:
You could use rbd info <volume_name>  to see the block_name_prefix, the object name consist like <block_name_prefix>.<sequence_number>,  so for example, rb.0.ff53.3d1b58ba.00000000e6ad should be the <e6ad>th object  of the volume with block_name_prefix rb.0.ff53.3d1b58ba.

     $ rbd info huge
        rbd image 'huge':
         size 1024 TB in 268435456 objects
         order 22 (4096 kB objects)
         block_name_prefix: rb.0.8a14.2ae8944a
         format: 1

-----Original Message-----
From: ceph-users [mailto:ceph-users-bounces at lists.ceph.com<javascript:;>] On Behalf Of Edwin Peer
Sent: Monday, January 5, 2015 3:55 AM
To: ceph-users at lists.ceph.com<javascript:;>
Subject: Re: [ceph-users] rbd resize (shrink) taking forever and a day

Also, which rbd objects are of interest?

<snip>
ganymede ~ # rados -p client-disk-img0 ls | wc -l
1672636
</snip>

And, all of them have cryptic names like:

rb.0.ff53.3d1b58ba.00000000e6ad
rb.0.6d386.1d545c4d.000000011461
rb.0.50703.3804823e.000000001c28
rb.0.1073e.3d1b58ba.00000000b715
rb.0.1d76.2ae8944a.00000000022d

which seem to bear no resemblance to the actual image names that the rbd command line tools understands?

Regards,
Edwin Peer

On 01/04/2015 08:48 PM, Jake Young wrote:
>
>
> On Sunday, January 4, 2015, Dyweni - Ceph-Users
> <6EXbab4FYk8H at dyweni.com<javascript:;> <mailto:6EXbab4FYk8H at dyweni.com<javascript:;>>> wrote:
>
>     Hi,
>
>     If its the only think in your pool, you could try deleting the
>     pool instead.
>
>     I found that to be faster in my testing; I had created 500TB when
>     I meant to create 500GB.
>
>     Note for the Devs: I would be nice if rbd create/resize would
>     accept sizes with units (i.e. MB GB TB PB, etc).
>
>
>
>
>     On 2015-01-04 08:45, Edwin Peer wrote:
>
>         Hi there,
>
>         I did something stupid while growing an rbd image. I accidentally
>         mistook the units of the resize command for bytes instead of
>         megabytes
>         and grew an rbd image to 650PB instead of 650GB. This all happened
>         instantaneously enough, but trying to rectify the mistake is
>         not going
>         nearly as well.
>
>         <snip>
>         ganymede ~ # rbd resize --size 665600 --allow-shrink
>         client-disk-img0/vol-x318644f-0
>         Resizing image: 1% complete...
>         </snip>
>
>         It took a couple days before it started showing 1% complete
>         and has
>         been stuck on 1% for a couple more. At this rate, I should be
>         able to
>         shrink the image back to the intended size in about 2016.
>
>         Any ideas?
>
>         Regards,
>         Edwin Peer
>         _______________________________________________
>         ceph-users mailing list
>         ceph-users at lists.ceph.com<javascript:;>
>         http://lists.ceph.com/listinfo.cgi/ceph-users-ceph.com
>
>     _______________________________________________
>     ceph-users mailing list
>     ceph-users at lists.ceph.com<javascript:;>
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>
>
> You can just delete the rbd header. See Sebastien's excellent blog:
>
> http://www.sebastien-han.fr/blog/2013/12/12/rbd-image-bigger-than-your
> -ceph-cluster/
>
> Jake
>
>
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Sorry, I misunderstood.

The simplest approach to me is to make another image of the correct size and copy your VM's file system to the new image, then delete the old one.

The safest thing to do would be to mount the new file system from the VM and do all the formatting / copying from there (the same way you'd move a physical server's root disk to a new physical disk)

I would not attempt to hack the rbd header. You open yourself up to some unforeseen problems.

Unless one of the ceph developers can comment there is a safe way to shrink an image, assuming we know that the file system has not grown since growing the disk.

Jake
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