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X-MS-Exchange-CrossTenant-Id: 4e2c6054-71cb-48f1-bd6c-3a9705aca71b X-MS-Exchange-CrossTenant-MailboxType: HOSTED X-MS-Exchange-CrossTenant-UserPrincipalName: 2V6dzgB6swpywFROGbL6lsIJhqYEBj+/4p4dlvsjlit/U9xqCrRWNG3y5oufbUBw6kaY+nKmPxSOnKdEa3xh/w== X-MS-Exchange-Transport-CrossTenantHeadersStamped: IA0PR10MB7546 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=2025-01-30_06,2025-01-30_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-2501300109 X-Proofpoint-ORIG-GUID: I2_CP-w26RksdwSJp6j-mPkAAVH2VQKr X-Proofpoint-GUID: I2_CP-w26RksdwSJp6j-mPkAAVH2VQKr On 29/01/2025 16:06, Ojaswin Mujoo wrote: > On Wed, Jan 29, 2025 at 08:59:15AM +0000, John Garry wrote: >> 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://urldefense.com/v3/__https://lore.kernel.org/linux-xfs/20241212013433.GC6678@frogsfrogsfrogs/__;!!ACWV5N9M2RV99hQ!IuMiPMbR5L3B8f31W8tbRlB7d0dMLg2nxW8k7KOGF3t031T99wahnbwnIeDn6N3AdveQJvmbL4V_FBwB0T9U9Q$ >> 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://urldefense.com/v3/__https://lore.kernel.org/linux-xfs/20240924061719.GA11211@lst.de/__;!!ACWV5N9M2RV99hQ!IuMiPMbR5L3B8f31W8tbRlB7d0dMLg2nxW8k7KOGF3t031T99wahnbwnIeDn6N3AdveQJvmbL4V_FBwv-uf6Ig$ >> > > Hey John, > > Thanks for taking a look. I did go through the 2 series sometime back. > I agree that there are some open challenges in getting the multi block > atomic write interface correct especially for mixed mappings and this is > one of the main reasons we want to explore the exchange_range fallback > in case blocks are not aligned. Right, so for XFS I am looking at a CoW-based fallback for unaligned/mixed mapping atomic writes. I have no idea on how this could work for ext4. > > That being said, I believe forcealign as a feature still holds a lot > of relevance as: > > 1. Right now, it is the only way to guarantee aligned blocks and hence > gurantee that our atomic writes can always benefit from hardware atomic > write support. IIUC DBs are not very keen on losing out on performance > due to some writes going via the software fallback path. Sure, we need performance figures for this first. > > 2. Not all FSes support COW (major example being ext4) and hence it will > be very difficult to have a software fallback incase the blocks are > not aligned. Understood > > 3. As pointed out in [1], even with exchange_range there is still value > in having forcealign to find the new blocks to be exchanged. Yeah, again, we need performance figures. For my test case, I am trying 16K atomic writes with 4K FS block size, so I expect the software fallback to not kick in often after running the system for a while (as eventually we will get an aligned allocations). I am concerned of prospect of heavily fragmented files, though. > > I agree that forcealign is not the only way we can have atomic writes > work but I do feel there is value in having forcealign for FSes and > hence we should have a discussion around it so we can get the interface > right. > I thought that the interface for forcealign according to the candidate xfs implementation was quite straightforward. no? What was not clear was the age-old issue of how to issue an atomic write of mixed extents, which is really an atomic write issue. > Just to be clear, the intention of this proposal is to mainly discuss > forcealign as a feature. I am hoping there would be another different > proposal to discuss atomic writes and the plethora of other open > challenges there ;) Thanks, John