[Discuss] LVM Snapshot Wrapup

[markw at mohawksoft.com (markw at mohawksoft.com)]

I'm sure everyone here is tired of the endless LVM discussions, but I
wanted to post a small summation.

In my research into LVM, I can state a few things I learned.

Snapshot Performance:
LVM snapshots are pretty heavy weight, but they are not as bad as
suggested by common benchmarks people post. Yes, upon the initial
"copy-on-write" (COW) operation the is a performance penalty, but this is
usually negligible over a period of time as it only occurs once per
"chunk."

Performance impact can be mitigated by using larger chunk sizes. The
default is 4K and that's pretty poor. Depending on your needs, a larger
chunk size will improve performance even in worst case scenarios. A 64K
chunk is a good balance between space usage and performance, 1/16 the
amount of work of a 4K chunk and still pretty manageable.

Differential Backups:
Differential backup of block devices can be accomplished with two
snapshots. A program like dds will accomplish this.

A few systems implement differential backup and replication using programs
that read the COW device of a snapshot to extract the change block
exception list. This operation renders the COW device inactive, so it is
best to delete the older snapshot immediately after the operation.

Pros
LVM works now
LVM is available in most distributions
LVM is amazingly stable and mature
ZFS has license issues.
Btrfs not considered "stable" yet.
Being a block driver, allows any filesystem or access like iSCSI or LUN
emulation.

Cons
LVM Snapshots are heavy weight and do not share data.
ZFS and Btrfs are considered better, regardless if they are or not, it
takes some focus away from LVM.
LVM too old to be cool