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Re: [Cluster-devel] [PATCH 2/4] mkfs.gfs2: Align resource groups to RAID stripes



On 06/06/13 13:57, Bob Peterson wrote:
Hi,

| +			/* Squeeze the last 1 or 2 rgs into the remaining space */
| +			if ((nextaddr < sdp->device.length) && (sdp->device.length - nextaddr >=
| minrgsz)) {
| +				rglen = sdp->device.length - nextaddr;
| +			} else {
| +				if (sdp->device.length - rgaddr <= maxrgsz)
| +					rgt->length = sdp->device.length - rgaddr;
| +				else
| +					rgt->length = maxrgsz;
| +				/* This is the last rg */
| +				nextaddr = 0;

In GFS1, we allowed mix-and-match resource group sizes, but we originally
designed mkfs.gfs2 to ensure that all rgrps were the same uniform size. This
usually means some space is wasted at the end of the last resource group.

We did this primarily so that fsck.gfs2 could more easily detect and repair
damaged resource groups and rindex values. At the time it was designed, I got
the buy-in of a bunch of developers and we all agreed to it. Since that time,
I've had to change fsck.gfs2 to take more drastic measures to repair damaged
resource groups, due to the fact that gfs2_convert can convert a GFS1 file
system to GFS2, and thus, we can still end up with non-uniform resource groups.
Many customers were adding storage and doing multiple gfs_grow ops,
which resulted in metadata sets where the rgrps and rindex were complete chaos.

Still, my assumption has always been: If the file system was made by
mkfs.gfs2, all resource groups (but the first one) are identical in size.

I think gfs2_grow takes some steps to ensure that new rgrps are also created
using the same size as the current resource groups. If we don't enforce
that rule, the rindex could once again become chaos, which means our chances
of rgrp and rindex repair get worse.

Do we still want to enforce this rule?

Good question. I had assumed that we don't have a rule like that as the rindex specifies the rg sizes. My next planned mkfs change is to allow the journal creation code to ask for a resource group large enough to contain all of a journal's data blocks so that they're always a single extent. Returning to enforcing the rule would have implications for that plan, too.

Andy

With the improved rgrp repair algorithms in fsck.gfs2, it may not be
necessary anymore. I'm not trying to be dogmatic; I'm looking for opinions here.

Regards,

Bob Peterson
Red Hat File Systems



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