Tweak a few checks to facilitate experimentation with an agcount=1 filesystem format with a larger agsize than the filesystem data size. The purpose of this is to POC a filesystem image mode format for XFS that better supports the typical cloud filesystem image deployment use case where a very small fs image is created and then immediately grown orders of magnitude in size once deployed to container environments. The large grow size delta produces filesystems with excessive AG counts, which leads to various other functional problems that eventually derive from this sort of pathological geometry. To experiment with this patch, format a small fs with something like the following: mkfs.xfs -f -lsize=64m -dsize=512m,agcount=1,agsize=8g <imgfile> Increase the underlying image file size, mount and grow. The filesystem will grow according to the format time AG size as if the AG was a typical runt AG on a traditional multi-AG fs. This means that the filesystem remains with an AG count of 1 until fs size grows beyond AG size. Since the typical deployment workflow is an immediate very small -> very large, one-time grow, the image fs can set a reasonable enough default or configurable AG size (based on user input) that ensures deployed filesystems end up in a generally supportable geometry (i.e. with multiple AGs for superblock redundancy) before seeing production workloads. Further optional changes are possible on the kernel side to help provide some simple guardrails against misuse of this mechanism. For example, the kernel could do anything from warn/fail or restrict runtime functionality for an insufficient grow. The image mode itself could set a backwards incompat feature bit that requires a mount option to enable full functionality (with the exception of growfs). More discussion is required to determine whether this provides a usable solution for the common cloud workflows that exhibit this problem and what the right interface and/or limitations are to ensure it is used correctly. Not-Signed-off-by: Brian Foster <bfoster@xxxxxxxxxx> --- Hi all, This is a followup to the idea Darrick brought up in the expansion discussion here [1]. I poked through the code a bit and found it somewhat amusing how little was in the way of experimenting with this, so threw this against an fstests run over the weekend. I see maybe around ~10 or so test failures, most of which look like simple failures related to either not expecting agcount == 1 fs' or my generally hacky/experimental changes. There are a couple or so that require a bit more investigation to properly characterize before I would consider this fully sane. I'm posting this separately from the expansion discussion to hopefully avoid further conflating the two. My current sense is that if this turns out to be a fundamentally workable approach, mkfs would more look something like 'mkfs --image-size 40g ...' and the kernel side may grow some optional guardrail logic mentioned above and in the previous discussion here [2], but others might have different ideas. Darrick, you originally raised this idea and then Eric brought up some legitimate technical concerns in the expansion design thread. I'm curious if either of you have any further thoughts/ideas on this. Brian [1] https://lore.kernel.org/linux-xfs/20240721230100.4159699-1-david@xxxxxxxxxxxxx/ [2] https://lore.kernel.org/linux-xfs/ZqzMay58f0SvdWxV@bfoster/ mkfs/xfs_mkfs.c | 11 +++++------ 1 file changed, 5 insertions(+), 6 deletions(-) diff --git a/mkfs/xfs_mkfs.c b/mkfs/xfs_mkfs.c index 6d2469c3c..50a874a03 100644 --- a/mkfs/xfs_mkfs.c +++ b/mkfs/xfs_mkfs.c @@ -325,8 +325,7 @@ static struct opt_params dopts = { }, .subopt_params = { { .index = D_AGCOUNT, - .conflicts = { { &dopts, D_AGSIZE }, - { &dopts, D_CONCURRENCY }, + .conflicts = { { &dopts, D_CONCURRENCY }, { NULL, LAST_CONFLICT } }, .minval = 1, .maxval = XFS_MAX_AGNUMBER, @@ -368,8 +367,7 @@ static struct opt_params dopts = { .defaultval = SUBOPT_NEEDS_VAL, }, { .index = D_AGSIZE, - .conflicts = { { &dopts, D_AGCOUNT }, - { &dopts, D_CONCURRENCY }, + .conflicts = { { &dopts, D_CONCURRENCY }, { NULL, LAST_CONFLICT } }, .convert = true, .minval = XFS_AG_MIN_BYTES, @@ -1233,7 +1231,7 @@ validate_ag_geometry( usage(); } - if (agsize > dblocks) { + if (agsize > dblocks && agcount != 1) { fprintf(stderr, _("agsize (%lld blocks) too big, data area is %lld blocks\n"), (long long)agsize, (long long)dblocks); @@ -2703,7 +2701,8 @@ validate_supported( * Filesystems should not have fewer than two AGs, because we need to * have redundant superblocks. */ - if (mp->m_sb.sb_agcount < 2) { + if (mp->m_sb.sb_agcount < 2 && + mp->m_sb.sb_agblocks <= mp->m_sb.sb_dblocks) { fprintf(stderr, _("Filesystem must have at least 2 superblocks for redundancy!\n")); usage(); -- 2.45.0
