On 11/09/2024 10:01, Ojaswin Mujoo wrote:
This patchset implements extsize hint feature for ext4. Posting this RFC to get
some early review comments on the design and implementation bits. This feature
is similar to what we have in XFS too with some differences.
extsize on ext4 is a hint to mballoc (multi-block allocator) and extent
handling layer to do aligned allocations. We use allocation criteria 0
(CR_POWER2_ALIGNED) for doing aligned power-of-2 allocations. With extsize hint
we try to align the logical start (m_lblk) and length(m_len) of the allocation
to be extsize aligned. CR_POWER2_ALIGNED criteria in mballoc automatically make
sure that we get the aligned physical start (m_pblk) as well. So in this way
extsize can make sure that lblk, len and pblk all are aligned for the allocated
extent w.r.t extsize.
Note that extsize feature is just a hinting mechanism to ext4 multi-block
allocator. That means that if we are unable to get an aligned allocation for
some reason, than we drop this flag and continue with unaligned allocation to
serve the request. However when we will add atomic/untorn writes support, then
we will enforce the aligned allocation and can return -ENOSPC if aligned
allocation was not successful.
A few questions/confirmations:
- You have no intention of adding an equivalent of forcealign, right?
- Would you also plan on using FS_IOC_FS(GET/SET)XATTR interface for
enabling atomic writes on a per-inode basis?
- Can extsize be set at mkfs time?
- Is there any userspace support for this series available?
- how would/could extsize interact with bigalloc?
Comparison with XFS extsize feature -
=====================================
1. extsize in XFS is a hint for aligning only the logical start and the lengh
of the allocation v/s extsize on ext4 make sure the physical start of the
extent gets aligned as well.
note that forcealign with extsize aligns AG block also
only for atomic writes do we enforce the AG block is aligned to physical
block
2. eof allocation on XFS trims the blocks allocated beyond eof with extsize
hint. That means on XFS for eof allocations (with extsize hint) only logical
start gets aligned. However extsize hint in ext4 for eof allocation is not
supported in this version of the series.
3. XFS allows extsize to be set on file with no extents but delayed data.
However, ext4 don't allow that for simplicity. The user is expected to set
it on a file before changing it's i_size.
4. XFS allows non-power-of-2 values for extsize but ext4 does not, since we
primarily would like to support atomic writes with extsize.
5. In ext4 we chose to store the extsize value in SYSTEM_XATTR rather than an
inode field as it was simple and most flexible, since there might be more
features like atomic/untorn writes coming in future.
6. In buffered-io path XFS switches to non-delalloc allocations for extsize hint.
The same has been kept for EXT4 as well.
Some TODOs:
===========
1. EOF allocations support can be added and can be kept similar to XFS
Note that EOF alignment for forcealign may change - it needs to be
discussed further.
Thanks,
John
.
Rest of the design details can be found in the individual commit messages.
Thoughts and suggestions are welcome!
Ojaswin Mujoo (5):
ext4: add aligned allocation hint in mballoc
ext4: allow inode preallocation for aligned alloc
ext4: Support for extsize hint using FS_IOC_FS(GET/SET)XATTR
ext4: pass lblk and len explicitly to ext4_split_extent*()
ext4: Add extsize hint support
fs/ext4/ext4.h | 12 +-
fs/ext4/ext4_jbd2.h | 15 ++
fs/ext4/extents.c | 224 ++++++++++++++----
fs/ext4/inode.c | 442 +++++++++++++++++++++++++++++++++---
fs/ext4/ioctl.c | 119 ++++++++++
fs/ext4/mballoc.c | 126 ++++++++--
fs/ext4/super.c | 1 +
include/trace/events/ext4.h | 2 +
8 files changed, 841 insertions(+), 100 deletions(-)
