Re: bitmaps on xfs

On 05/04/2021 18:46, David T-G wrote:

Wol & Roger, et al --

...and then antlists said...
%
% On 05/04/2021 12:30, Roger Heflin wrote:
% >>   diskfarm:~ # grep /mnt/ssd /etc/fstab
% >>   LABEL=diskfarm-ssd      /mnt/ssd        xfs     defaults        0  0
% >>
% >>will work for my bitmap files target, since all I see is that it must be
% >>an ext2 or ext3 (not ext4? old news?) device.
%
% Bear in mind you're better off using a journal (and bitmaps and
% journals are incompatible).

A journal of the filesystem (XFS or ReiserFS) on the RAID5 device?  Or a journal
of the actual md?

Journal of the md. I'm thinking raid journal, which fixes the raid-5 write hole (I don't understand it, but if a system crashes in the middle of a raid-5 write it can apparently mess things up something horrid).

   diskfarm:~ # df -kh /mnt/4Traid5md/ /mnt/750Graid5md/
   Filesystem      Size  Used Avail Use% Mounted on
   /dev/md0p1       11T   11T  309G  98% /mnt/4Traid5md
   /dev/md127p1    1.4T  1.4T   14G 100% /mnt/750Graid5md

   diskfarm:~ # mount | grep /dev/md
   /dev/md0p1 on /mnt/4Traid5md type xfs (rw,relatime,attr2,inode64,logbufs=8,logbsize=32k,sunit=1024,swidth=2048,noquota)
   /dev/md127p1 on /mnt/750Graid5md type reiserfs (rw,relatime)


%
...
% >I am about to do some array reworks to go from all 3tb disks to start
% >using some 6tb disks.   If the file was pre-allocated I would not
...
% >
% Umm... don't use all the space on your 6TB disks. I'm planning to
% build my arrays on dm-integrity, which will make raid 5 a bit more
% trustworthy.
[snip]

Oooh, something else to learn :-)  I hope to go from 4 drives to 6 when I
do, and I'll be buying the best GB/$ at the time, but it will also be a
grow-over-time thing.

dm-integrity is nothing to do with raid per-se, but it does a checksum of the data on disk. If your data is corrupted (rather than lost) there's no way you can get it back with raid-5. dm-integrity turns corruption into data loss allowing raid-5 to recover.

Read my journey building my new system ... :-)
https://raid.wiki.kernel.org/index.php/System2020

I've got a little more to add, but it's stalled for the perennial problem of finding time to do and concentrate.

Cheers,
Wol