Hi,
I would like to solve an issue that LVM mirror can use only one
mirror log. I hope this post would start good discussion and go
forward to solve it.
ISSUES
======
Current implementation allows LVM mirror to use only one log disk.
A log disk keeps a bitmap and a each bit in the bitmaps shows the
status (in-sync or out-of-sync) of a region among mirror legs.
When a mirror is activated, log disk is checked and the data in the
out-of-sync region is copied from primary mirror device to others.
However, once a log disk gets broken, and the data in the log disk
can not be read, data replications for a whole disk are required
when the mirror is activated since LVM mirror (dm-mirror) can not
decide which regions are in-sync.
If mirror disk size is huge, it takes long time for disk replication,
and it utilizes much system resources, such as I/O bandwidth, CPU,
memory. That would cause system performance degradation until disk
replication completes. A log disk failure at boot is highly possible,
and the whole disk replication should be avoided.
PROPOSED APPROACH
=================
We have discussed two approaches to configure more than one log
disk so far.
- dm-log: support multi-log devices
https://www.redhat.com/archives/dm-devel/2008-November/msg00113.html
- lvm2: mirroredlog support (posted by malahal)
https://www.redhat.com/archives/dm-devel/2008-December/msg00095.html
The first approach, multi-log, is a kernel side approach and can
do quick disk blockage inside the kernel, while mirroredlog uses
current mirroring feature and constructs a log disk as a mirrored
disk.
I think that the mirroredlog approach is a quick and easy way to
achieve redundant mirror log. I would like to proceed the mirroredlog
approach so that it is merged into upstream.
I appreciate your comments.
Thanks,
---
Takahiro Yasui
Hitachi Computer Products (America) Inc.
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