Re: [Qemu-devel] [RFC] qed: Add QEMU Enhanced Disk format

[Avi Kivity]

 On 09/10/2010 04:22 PM, Anthony Liguori wrote:
> > Looks like it depends on fsck, which is not a good idea for large 
> > images.
>
>
> fsck will always be fast on qed because the metadata is small.  For a 
> 1PB image, there's 128MB worth of L2s if it's fully allocated

It's 32,000 seeks.

> (keeping in mind, that once you're fully allocated, you'll never fsck 
> again).

Why?  Fully populated L1 (so all L2s are allocated) doesn't mean a fully 
allocated image.  You're still allocating and linking into L2s.

>   If you've got 1PB worth of storage, I'm fairly sure you're going to 
> be able to do 128MB of reads in a short period of time.  Even if it's 
> a few seconds, it only occurs on power failure so it's pretty reasonable.

Consider a cloud recovering from power loss, even if you're fscking 
thousands of 100GB images you'll create a horrible seek storm on your 
storage (to be followed by a seek storm from all the guests booting).

No, fsck is not a good idea.

> > > I need to look at the actual ATA and SCSI specs for how this will
> > > work.  The issue I am concerned with is sub-cluster trim operations.
> > > If the trim region is less than a cluster, then both qed and qcow2
> > > don't really have a way to handle it.  Perhaps we could punch a hole
> > > in the file, given a userspace interface to do this, but that isn't
> > > ideal because we're losing sparseness again.
> >
> > To deal with a sub-cluster TRIM, look at the surrounding sectors.  If 
> > they're zero, free the cluster.  If not, write zeros or use 
> > sys_punch() to the range specified by TRIM.
>
> Better yet, if you can't trim a full cluster, just write out zeros and 
> have a separate background process that punches out zero clusters.

That can work as well, or a combination perhaps.

> That approach is a bit more generic and will help compact images 
> independently of guest trims.

You still need a freelist.

--
I have a truly marvellous patch that fixes the bug which this
signature is too narrow to contain.