[linux-lvm] Very slow i/o after snapshotting
mickylmartin at gmail.com
Tue Jul 9 14:04:10 UTC 2013
> Do you write to the snapshot ?
Not so often but there is like 1-5% usage allocation.
> It's known FACT that performance of old snapshot is very far from being
> ideal - it's very simply implementation - for a having consistent system to
> make a backup of the volume - so for backup it doesn't really matter how
> slow is that (it just needs to remain usable)
True. But in case of domains running on a hypervisor, the purpose of doing
a live backup slingshots and dies! I know it's not LVM's fault but
> I'd suggest to go with much smaller chunks - i.e. 4-8-16KB - since if you
> update a single 512 sector - 512KB of data has to be copied!!! so really
> bad idea, unless you overwrite large continuous portion of a device.
I just tried that and got 2-3% improvement.
Here are the gritty details, if someone's interested.
--- Logical volume ---
LV Write Access read/write
LV snapshot status active destination for lvma
LV Status available
# open 1
LV Size 200.10 GiB
Current LE 51226
COW-table size 100.00 GiB
COW-table LE 25600
Allocated to snapshot 0.14%
Snapshot chunk size 4.00 KiB
Read ahead sectors auto
- currently set to 16384
Block device 253:26
> And yes - if you have rotational hdd - you need to expect horrible seek
> times as well when reading/writing from snapshot target....
Yes, they do. But I reproduced this one with multiple machines (and kernels)!
> And yes - there are some horrible Segate hdd drives (as I've seen just
> yesterday) were 2 disk reading programs at the same time may degrade 100MB/s
> -> 4MB/s (and there is no dm involved)
Haha, no doubt. Seagates' are the worst ones. IMHO, Hitachi's drives
run cooler and
that's what Nagios tells me!
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