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bffcea87-972b-49d4-2580-08dd40433784 X-MS-Exchange-CrossTenant-AuthSource: DM6PR10MB4313.namprd10.prod.outlook.com X-MS-Exchange-CrossTenant-AuthAs: Internal X-MS-Exchange-CrossTenant-OriginalArrivalTime: 29 Jan 2025 08:59:19.7226 (UTC) X-MS-Exchange-CrossTenant-FromEntityHeader: Hosted X-MS-Exchange-CrossTenant-Id: 4e2c6054-71cb-48f1-bd6c-3a9705aca71b X-MS-Exchange-CrossTenant-MailboxType: HOSTED X-MS-Exchange-CrossTenant-UserPrincipalName: lAMVIimXnDYFDqInJi1UHfD7ZACgUrRd356AP+rSFKgQ/aU7OgtieBAdrGm2F4ZfDjK50nXGz6yErrYbh8/2Pw== X-MS-Exchange-Transport-CrossTenantHeadersStamped: SN7PR10MB6644 X-Proofpoint-Virus-Version: vendor=baseguard engine=ICAP:2.0.293,Aquarius:18.0.1057,Hydra:6.0.680,FMLib:17.12.68.34 definitions=2025-01-28_04,2025-01-27_01,2024-11-22_01 X-Proofpoint-Spam-Details: rule=notspam policy=default score=0 suspectscore=0 mlxscore=0 spamscore=0 malwarescore=0 mlxlogscore=999 bulkscore=0 phishscore=0 adultscore=0 classifier=spam adjust=0 reason=mlx scancount=1 engine=8.12.0-2411120000 definitions=main-2501290072 X-Proofpoint-GUID: BsF8fkWL2GUDpEvcTd5ODZlMTScBsJRM X-Proofpoint-ORIG-GUID: BsF8fkWL2GUDpEvcTd5ODZlMTScBsJRM On 29/01/2025 07:06, Ojaswin Mujoo wrote: Hi Ojaswin, > > I would like to submit a proposal to discuss the design of extsize and > forcealign and various open questions around it. > > ** Background ** > > Modern NVMe/SCSI disks with atomic write capabilities can allow writes to a > multi-KB range on disk to go atomically. This feature has a wide variety of use > cases especially for databases like mysql and postgres that can leverage atomic > writes to gain significant performance. However, in order to enable atomic > writes on Linux, the underlying disk may have some size and alignment > constraints that the upper layers like filesystems should follow. extsize with > forcealign is one of the ways filesystems can make sure the IO submitted to the > disk adheres to the atomic writes constraints. > > extsize is a hint to the FS to allocate extents at a certian logical alignment > and size. forcealign builds on this by forcing the allocator to enforce the > alignment guarantees for physical blocks as well, which is essential for atomic > writes. > > ** Points of discussion ** > > Extsize hints feature is already supported by XFS [1] with forcealign still > under development and discussion [2]. From https://lore.kernel.org/linux-xfs/20241212013433.GC6678@frogsfrogsfrogs/ thread, the alternate solution to forcealign for XFS is to use a software-emulated fallback for unaligned atomic writes. I am looking at a PoC implementation now. Note that this does rely on CoW. There has been push back on forcealign for XFS, so we need to prove/disprove that this software-emulated fallback can work, see https://lore.kernel.org/linux-xfs/20240924061719.GA11211@lst.de/ > After taking a look at ext4's multi-block > allocator design, supporting extsize with forcealign can be done in ext4 as > well. There is a RFC proposed which adds support for extsize hints feature in > ext4 [3]. However there are some caveats and deviations from XFS design. With > these in mind, I would like to propose LSFMM topic on: > > * exact semantics of extsize w/ forcealign which can bring a consistent > interface among ext4 and xfs and possibly any other FS that plans to > implement them in the future. > > * Documenting how forcealign with extsize should behave with various FS > operations like fallocate, truncate, punch hole, insert/collapse range etc > > * Implementing extsize with delayed allocation and the challenges there. > > * Discussing tooling support of forcealign like how are we planning to maintain > block alignment gurantees during fsck, resize and other times where we might > need to move blocks around? > > * Documenting any areas where FSes might differ in their implementations of the > same. Example, ext4 doesn't plan to support non power of 2 extsizes whereas > XFS has support for that. > > Hopefully this discussion will be relevant in defining consistent semantics for > extsize hints and forcealign which might as well come useful for other FS > developers too. > > Thoughts and suggestions are welcome. > > References: > [1] https://urldefense.com/v3/__https://man7.org/linux/man-pages/man2/ioctl_xfs_fsgetxattr.2.html__;!!ACWV5N9M2RV99hQ!NoUXCJI_ofztyeV6aq2HvNI4YHcyjSHvzxHkw0fSGB9_SKz6jkAqzBVy7WcUSNNHrJl0jM0qolbvuVK2oQKuYw$ > [2] https://urldefense.com/v3/__https://lore.kernel.org/linux-xfs/20240813163638.3751939-1-john.g.garry@oracle.com/__;!!ACWV5N9M2RV99hQ!NoUXCJI_ofztyeV6aq2HvNI4YHcyjSHvzxHkw0fSGB9_SKz6jkAqzBVy7WcUSNNHrJl0jM0qolbvuVLgqkSeIg$ > [3] https://urldefense.com/v3/__https://lore.kernel.org/linux-ext4/cover.1733901374.git.ojaswin@linux.ibm.com/__;!!ACWV5N9M2RV99hQ!NoUXCJI_ofztyeV6aq2HvNI4YHcyjSHvzxHkw0fSGB9_SKz6jkAqzBVy7WcUSNNHrJl0jM0qolbvuVJ_GK50Cg$ > > Regards, > ojaswin