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Oracle suggests to have the external journal on a low latency device.

The default location for an XFS journal is on the same block device as the data. As synchronous metadata writes to the journal must complete successfully before any associated data writes can start, such a layout can lead to disk contention for the typical workload pattern on a database server. To overcome this problem, you can place the journal on a separate physical device with a low-latency I/O path.

The device with the lowest latency is a RAM disk. But is it possible to use a RAM disk?

I ask because it seems to be necessary to specify the logdev option during the creation of the XFS filesystem. Any structure written to the logdev during XFS creation will be lost after a reboot, if I put it on a RAM disk.

I know the RAM disk will not help against power loose. But my use-case is a SSD connected by USB. I just want some protection against pulling the USB cable.

Out of the box it does not seem to work.

Create 100 MB RAM-Disk in /dev/ram0.

# modprobe brd rd_size=102400 max_part=1 rd_nr=1

Create XFS with external log.

# mkfs.xfs -l logdev=/dev/ram0 /dev/sdc1

Mount filesystem.

# mkdir /media/xfs
# mount -t xfs -o logdev=/dev/ram0 /dev/sdc1 /media/xfs

Unmount.

# umount /media/xfs

Remove RAM disk.

# rmmod brd

Recreate RAM disk.

# modprobe brd rd_size=102400 max_part=1 rd_nr=1

Remount filesystem.

# mount -t xfs -o logdev=/dev/ram0 /dev/sdc1 /media/xfs
mount: /media/xfs: wrong fs type, bad option, bad superblock on /dev/sdc1, missing codepage or helper program, or other error.

But when I repair the filesystem, I can mount it.

# xfs_repair -l /dev/ram0 /dev/sdc1
[...]
Phase 2 - using external log on /dev/ram0
        - zero log...
totally zeroed log
[...]
Maximum metadata LSN (1:2880) is ahead of log (0:0).
Format log to cycle 4.
done
# mount -t xfs -o logdev=/dev/ram0 /dev/sdc1 /media/xfs

Now I am wondering, if it is a good idea to do so.

ceving
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  • Possible? Yes. (Obviously you should persist the log somehow to avoid repairs - which might still fail depending on the state the filesystem is in). Even then it's unclear if it would help your use-case at all, maybe try the wsync option instead? – frostschutz Jun 22 '22 at 09:48

1 Answers1

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You can use zram, assume only one zram device and sda as the block device

$ modprobe zram num_devices=1
$ echo 1G > /sys/block/zram0/disksize
$ mkfs.xfs /dev/sda -L scratch01 -l logdev=/dev/zram0 -f -K -s size=4096
$ mount -L scratch01 -o logdev=/dev/zram0 /mnt

$ df -h /mnt/
Filesystem      Size  Used Avail Use% Mounted on
/dev/sda        746G  5.3G  740G   1% /mnt
Mike
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