Re: [linux-lvm] Oops when running snapshots

[Steve McIntyre <smcintyre@software.plasmon.com>]

On Tue, Jan 13, 2004 at 11:35:46AM -0700, Andrew Patterson wrote:
>
>We worked over this problem with Heinz on LVM 1.07.  One of our kernel
>hackers (along with Heinz) came up with several possible solutions to a
>race condition in the snapshot code.  They are presented below.  We
>found that option #1 worked, but sometimes it could take a long time to
>create multiple snapshots of the same LV under extremely heavy load. 
>The first snapshot is fine, but the second or more can take hours. 
>Option #2 created a huge performance penalty to writes on the original
>LV (90-99%).  I don't remember that option #3 helped any, and we never
>tried #4.  We settled with option #1 and solved the long snapshot
>creation time by stopping I/O while creating the snapshot.

Ok, thanks. We had another hard lockup overnight caused by this
problem. I've just applied #1; I'll let you know how it goes.

<snip>

>Options to fix (included as attachments and inline):
>
>1) don't do the hash table optimization in lvm_find_exception_table,
>which
>we tried last night.  No crashes, but slow.  Doesn't fix the "get the
>write
>semaphore while holding the read semaphore" problem in
>__remap_snapshot().
>
>--- lvm-snap.c~ Mon Aug 25 15:28:50 2003
>+++ lvm-snap.c-erik     Thu Aug 28 13:33:28 2003
>@@ -130,11 +130,13 @@ static inline lv_block_exception_t *lvm_
>                exception = list_entry(next, lv_block_exception_t,
>hash);
>                if (exception->rsector_org == org_start &&
>                    exception->rdev_org == org_dev) {
>+#if 0
>                        if (i) {
>                                /* fun, isn't it? :) */
>                                list_del(next);
>                                list_add(next, hash_table);
>                        }
>+#endif
>                        ret = exception;
>                        break;
>                }

-- 
Steve McIntyre, Plasmon                      smcintyre@software.plasmon.com
Getting a SCSI chain working is perfectly simple if you remember that there
must be exactly three terminations: one on one end of the cable, one on the
far end, and the goat, terminated over the SCSI chain with a silver-handled
knife whilst burning *black* candles. --- Anthony DeBoer