Date: Wed, 6 Sep 2006 15:34:57 +0200 From: Ulrich =?iso-8859-1?q?Sp=F6rlein?= <ulrich.spoerlein@1822direkt.com> To: stable@freebsd.org Subject: Snapshot duration, performance and how to avoid I/O lock Message-ID: <239947161@misc1.1822direkt.com>
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Hi,
I have to create regular snapshots of several volumes roughly 1.4TB in
size (each). But using mksnap_ffs takes a lot of time (45 minutes) and
it looks like it could be speed up.
iostat reports some 2MB/s of I/O
tty da0 da1 sa0
cpu
tin tout KB/t tps MB/s KB/t tps MB/s KB/t tps MB/s us ni sy in id
0 77 16.00 120 1.87 0.00 0 0.00 0.00 0 0.00 0 0 8 4 89
0 231 16.00 131 2.04 0.00 0 0.00 0.00 0 0.00 0 0 6 2 91
0 77 16.00 127 1.98 0.00 0 0.00 0.00 0 0.00 0 0 5 3 92
0 204 16.00 123 1.92 0.00 0 0.00 0.00 0 0.00 0 0 6 3 91
0 77 16.00 128 2.00 0.00 0 0.00 0.00 0 0.00 0 0 6 3 91
gstat reports
dT: 0.501s w: 0.500s
L(q) ops/s r/s kBps ms/r w/s kBps ms/w %busy Name
1 126 84 1341 11.8 42 671 2.7 99.7| da0
The filesystem under snapshotting was *empty*, and right now is at
Filesystem 1K-blocks Used Avail Capacity Mounted on
/dev/da0s2d 1376038286 815092 1265140132 0% /export/vol1
How come, a snapshot of an empty file system takes more than 800MB? Aren't the
blocks copy-on-write?
Why is the disk 100% busy while only a mere 2MB/s are pushed? This is on a
3ware device driver for 9000 series storage controllers, version: 3.60.02.012
twa0: <3ware 9000 series Storage Controller> port 0x3000-0x303f mem 0xdc000000-0xddffffff,0xd8300000-0xd8300fff irq 48 at device 1.0 on pci3
twa0: [FAST]
twa0: INFO: (0x15: 0x1300): Controller details:: Model 9550SX-8LP, 8 ports, Firmware FE9X 3.04.00.005, BIOS BE9X 3.04.00.002
with a RAID5 over 7 SATA disks.
da0 at twa0 bus 0 target 0 lun 0
da0: <AMCC 9550SX-8LP DISK 3.04> Fixed Direct Access SCSI-3 device
da0: 100.000MB/s transfers
da0: 1430448MB (2929557504 512 byte sectors: 255H 63S/T 182356C)
Another thing is blocking other disk I/O while snapshotting. Right now I did
a ls(1) in the .snap directory, so I understand the filesystem is now suspended.
The workaround would then be to "dont do that". But what if other snapshots are
accessed during that time? I want to provide yesterdays snapshot to our users
while taking the current snapshot and providing access to the newest data at the
same time.
Cheers
Ulrich Spoerlein
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