Re: [PATCH] [RFC] iomap: Use FUA for pure data O_DSYNC DIO writes

[Christoph Hellwig <hch@lst.de>]

> I think the answer is the iomap* logic makes sure to issue generic_write_sync for O_DSYNC W1 after W1 is completed by hardware and then waits for completion of the flush request (REQ_PREFLUSH) before W1 is returned to the AIO completion ring, preventing io_getevents from processing W1 before the flush occurs and completes.   I just need proper confirmation from the experts on this code that this is the expected behavior.

Yes. that is the case.

> For SQL Server using O_DIRECT | O_DSYNC on current kernels is very performance impacting.   Instead we enable a mode for SQL that opens O_DIRECT only and issues fsync/fdatasync when we are hardening log files or checkpointing data files.   This reduces the write, flush, write, flush pattern allowing for write, write, write, ... then flush as we only issue flush requests when required to maintain the data integrity boundaries of SQL Server.  The performance is significantly better then the device flush for each write as you can imagine.
> 
> Testing shows the FUA enhancement is better then the write, flush pattern.   For SQL Server we want to dynamically open with O_DIRECT | O_DSYNC when REQ_FUA can be properly used and open with O_DIRECT and leverage SQL Server's alternate flush scheme when running on older kernel or a system that does not support FUA (SATA, IDE, ...)

There is no really good way to figure this out except for benchmarking,
given that the FUA use an implementation detail.

Btw, another feature that might be interesting to you is the RWF_DSYNC
flag to the pwritev2 syscall, which allows to apply O_DSYNC semantics on
a per-I/O basis instead of using it at open time.