XFS: the filesystem of the future?

but shrinking should never fail in a way that leaves the filesystem invalid.

block numbers will need to change, but I don't see why inode numbers would have to change (and if you don't change those, then lots of other problems vanish), they are already independent of where on the disk the data lives.

this seems fairly obvious to me, what am I missing that makes the simple approach of

identify something to move
copy the data blocks
change the block pointers to the new blocks
free the old blocks
repeat until you have moved everything

not work? (at least for the file data)

if you try to do this on a live filesystem, then you need to do a lot of locking and other changes to make sure the data doesn't change under you (and that new data doesn't go into the space you are trying to free), but if the filesystem is offline for the shrink this shouldn't be an issue.

moving metadata will be more complex, but the worst case should be that something can't be moved, and so you can't shrink the filesystem beyond that point, but there should still be no risk.

I would not want to argue that shrinking support is actually needed and that was not my intention, though fstrim is certainly new to me and I thank you for that pointer. Let me explain more precisely my XFS problem in case you would like to understand more of the kinds of things that happen to people that discourage them from moving away from the familiar:

I was provisioning a new 8TB server (4 x 2TB physical disks) that was to serve as a storage area for a number of different systems. Some peculiarities of the applications meant that separate partitions were desirable. Overall, I needed a number of small (~50-250GB) volumes and to be able to utilise the remaining space for VM images. Since I wasn't sure about how many of the smaller partitions I needed, I used LVM to create one 8TB PV with LVs allocated upon that to suit.

When it came to creating the largest logical volume for all the remaining free space, I decided to try XFS because I had heard it was more efficient with large files and I was slightly worried about the performance impact of LVM (it was my first time playing with LVM also).

So I provisioned around 6TB that remained for the large XFS partition and soon filled it up. Then came the half-expected requirement to add another couple of smaller partitions. No problem I thought, LVM to the rescue. Or it would have been if I could have shrunk the XFS filesystem to truncate the logical volume!

I may well misunderstand the subtleties of XFS block allocations over LVM's physical extent mappings, sparse file allocation and the like for thin-provisioning (I often do misunderstand such things) but I don't think fstrim would have helped me there even had I known about it at the time.

I only had a 100Mbps NIC on the server so it took quite some time (days if I recall) to backup all that data, recreate the filesystem as ext4 and copy it all back.

It may well be that my use case was highly unusual, my research insufficient, my knowledge limited and/or my strategy idiotic. I'm a programmer first and reluctant sysadmin. But this is not at all unusual outside of such groups of experienced experts such as you'd find at LWN.

One reason for my posting about the shrinking issue was that I hadn't seen it mentioned yet (sorry, I did not watch the video of the full talk), but really my point was that I ended up causing myself a great deal of trouble which only re-enforced to me the wisdom of the tech dinosaurs I have worked with in my time that you should avoid deviating from what you know and trust, without sufficiently good reason.

There are many who take this view, at least enough that it seems improbable to me that you will succeed in convincing the partly-informed, risk-averse and time-constrained majority to displace ext4 for XFS as a default choice in their minds.

I think it's great that you are taking the time to promote the benefits of XFS and to keep improving on it. I read this article precisely because I was made aware through my previous screw-ups that I need to invest more time learning about different filesystems, but I generally feel better knowing about where I'm likely to face problems as much as what I have to gain. And for that reason I still find it useful when people pick up on minor detractions, even if they seem unimportant in the grand scheme of things.

Hope you understand!
-Will