[CentOS] 32-bit kernel+XFS+16.xTB filesystem = potential disaster (was:Re: ZFS @ centOS)
lowen at pari.edu
Tue Apr 5 17:21:34 UTC 2011
On Monday, April 04, 2011 11:09:29 PM Warren Young wrote:
> I did this test with Bonnie++ on a 3ware/LSI 9750-8i controller, with
> eight WD 3 TB disks attached. Both tests were done with XFS on CentOS
> 5.5, 32-bit. (Yes, 32-bit. Hard requirement for this application.)
> For the RAID-6 configuration, I used the 3ware card's hardware RAID,
> creating a single ~16 TB volume, formatted XFS.
> Dropping to 16.37 TB on the RAID configuration by switching
> to RAID-6 let us put almost the entire array under a single 16 TB XFS
You really, really, really don't want to do this. Not on 32-bit. When you roll one byte over 16TB you will lose access to your filesystem, silently, and it will not remount on a 32-bit kernel. XFS works best on a 64-bit kernel for a number of reasons; the one you're likely to hit first is the 16TB hard limit for *occupied* file space; you can mkfs an XFS filesystem on a 17TB or even larger partition or volume, but the moment the occupied data rolls over the 16TB boundary you will be in disaster recovery mode, and a 64-bit kernel will be required for rescue.
The reason I know this? I had it happen. On a CentOS 32-bit backup server with a 17TB LVM logical volume on EMC storage. Worked great, until it rolled 16TB. Then it quit working. Altogether. /var/log/messages told me that the filesystem was too large to be mounted. Had to re-image the VM as a 64-bit CentOS, and then re-attached the RDM's to the LUNs holding the PV's for the LV, and it mounted instantly, and we kept on trucking.
There's a reason upstream doesn't do XFS on 32-bit.
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