I have a ZFS filesystem whose underlying pool is a RAID1 mirror of two spinning-rust drives. The drives are quite new and I bet they really have 4k sectors, but they report themselves as having 512-byte sectors and because I'm an idiot I didn't do anything about this when I set up the system. As a result, my mirror vdev has ashift=9.
This seems like it may be bad for two reasons.
So:
A few more details, in case they're relevant. I'm running FreeBSD 9.1 on an amd64 system. It has 4GB of RAM. (At some point I will surely increase that; is it urgent?) My existing setup has a zpool containing just one vdev, a RAID1 mirror of two 2TB hard drives. The device I intend to use for L2ARC is an Intel 520-series SSD, 120GB in size.
Many thanks in advance for any advice!
This seems like it may be bad for two reasons.
- Presumably ZFS will be doing lots of 512-byte I/O operations on the drive, and they will all really be reading or writing 4096 bytes at a time, so everything will be less efficient than it should be.
- I am about to add an SSD as an L2ARC device; my hazy understanding is that getting the sizes and alignments right is especially important for SSDs, and I have a vague fear (not backed by any concrete evidence) that this will be harder if my existing storage is all using ashift=9 and the SSD wants everything 4k-aligned.
So:
- Can I somehow make all my existing storage use ashift=12? If so, how? Answers that don't begin "first, transfer all your data onto another device because we're going to blow it all away" would be particularly welcome, because I am easily scared .
- Suppose for the sake of argument that I can't, or that I don't even though I can; so everything in my pool is still on a vdev with ashift=9. Can I / should I / need I do anything special when setting up my new SSD as an L2ARC device, to make sure that I/O operations done on it are properly aligned?
A few more details, in case they're relevant. I'm running FreeBSD 9.1 on an amd64 system. It has 4GB of RAM. (At some point I will surely increase that; is it urgent?) My existing setup has a zpool containing just one vdev, a RAID1 mirror of two 2TB hard drives. The device I intend to use for L2ARC is an Intel 520-series SSD, 120GB in size.
Many thanks in advance for any advice!