I have recently started using LVM on some servers for hard drives larger than 1 TB. They're useful, expandable and quite easy to install. However, I could not find any data about the dangers and caveats of LVM.
What are the downsides of using LVM?
There are numerous questions on Serverfault about aligning 4k sectors disks, but one thing is not really clear to me yet.
I successfully aligned my RAID1+LVM. One of the things I did was use mdadm superblock version 1.0 (which stores the superblock at the end of the disk).
The manpage says this:
The different sub-versions store the superblock at different locations on the device, either at the end (for 1.0), at the start (for 1.1) or 4K from the start (for 1.2). "1" is equivalent to "1.0". "default" is equivalent to "1.2".
Is the 1.2 version, which is default, made for 4k sectors drives? The way I see it, it is not, because 4k from the start + the length of the superblock is not a multitude of 4k (the superblock is about 200 bytes long, if I remember correctly).
Any insight into this is welcome.
edit:
below was answered that mdadm superblock 1.1 and 1.2 are meant for 4k alignment. I just created a whole-device raid with:
mdadm --create /dev/md4 -l 1 -n 2 /dev/sdb /dev/sdd
Then I added a logical volume to it:
vgcreate universe2 /dev/md4
The array is syncing at 16 MB/s:
md4 : active raid1 sdd[1] sdb[0]
1465137424 blocks super 1.2 [2/2] [UU]
[>....................] resync = 0.8% (13100352/1465137424) finish=1471.6min speed=16443K/sec
So I doubt it is properly aligned.
(disks are 1.5 TB WD EARS. I have them in my desktop PC and they synced at about 80 MB/s.)
Edit2:
Here's --examine output:
# mdadm --examine /dev/sdb
/dev/sdb:
Magic : a92b4efc
Version : 1.2
Feature Map : 0x0
Array UUID : 79843828:7d939cce:1c8f0b32:cf339870
Name : brick:4 (local to host brick)
Creation Time : Sat Jul 9 10:47:33 2011
Raid Level : raid1
Raid Devices : 2
Avail Dev Size : 2930275120 (1397.26 GiB 1500.30 GB)
Array Size : 2930274848 (1397.26 GiB 1500.30 GB)
Used Dev Size : 2930274848 (1397.26 GiB 1500.30 GB)
Data Offset : 2048 sectors
Super Offset : 8 sectors
State : active
Device UUID : dd2e3b5f:33214b96:1cb88169:25deb050
Update Time : Sat Jul 9 10:49:06 2011
Checksum : 4f7cd785 - correct
Events : 1
Device Role : Active device 0
Array State : AA ('A' == active, '.' == missing)
Data offset is 2048 sectors, which is dividable by 8, so one would think it's ok. The volume group has a physical extent size of 4 MiB, which is also dividable by 8. But that wouldn't even matter, because the resync is not related to what the device contains.
Another edit: it doesn't appear to be an alignment issue; since hdparm -t shows a very low read speed for one of the disks (30 MB/s). Something else is amiss.
Edit2: I never remember to update this post when I found the answer. All is nicely aligned. One of the disks was broken. Apparently it was on its last leg and even that broke at some point. A replacement disk worked fine.
I have a storage system that contains 8 x 1TB drives that use the 4k sector size "Advanced Format". I'm planning to run NexentaStor on this hardware and want to ensure that I'm taking the 4k sector size into account. Is there anything special I need to keep in mind when creating the root pool and subsequent data pools with ZFS?
I know that I have to align my 4k drives by a multiple of 8 sectors, but what about md-RAID / LVM / dm-crypt? How do I tell these layers that my drive is 4k? If they don't respect the 4k sector size, the partition alignment is useless. How do I align LVM/md/crypto-layers? Thanks.
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