> > 1. Most drives do NOT respond to CMD_FLUSH immediately i.e. they wait > until the data is actually moved to the non-volatile media (which is > the right behaviour) i.e. performance drops. Which is what the specification sais they must do. > 2. Some drives may implement CMD_FLUSH to return immediately i.e. no > guarantee the data is actually on disk. In which case they aren't spec complicant. While I've seen countless data integrity bugs on lower end ATA SSDs I've not seen one that simpliy ingnores flush. If you'd want to cheat that bluntly you'd be better of just claiming to not have a writeback cache. > 3. Anyway, CMD_FLUSH does NOT guarantee atomicity. (Consider power > failure in the middle of an ongoing CMD_FLUSH on non battery-backed > disks). It does not guarantee atomicy by itself, but it's the only low-level primitive a filesystem or database can use build atomic transaction at a higher level on an ATA disk with the writeback cache enabled. > In case the application cannot be modified to perform ASYNC IO, there > exists a way to disable the behaviour of issuing a CMD_FLUSH for each > sync() within the block device driver for SATA/SCSI disks. This is > what is described by > https://gist.github.com/TheCodeArtist/93dddcd6a21dc81414ba Which is utterly broken, and your insistance on pushing it shows you do not understand the problem space. You solve your performance problem by completely disabling any chance of having data integrity guarantees, and do so in a way that is not detectable for applications or users. If you have a workload with lots of small synchronous writes disabling the writeback cache on the disk does indeed often help, especially with the non-queueable FLUSH on all but the most recent ATA devices. If you do have workloads where you do lots of synchronous writes > Just to be clear, i am NOT recommending that this change be mainlined; > rather it is a reference to improve performance in the rare cases(like > in the OP Stefan's case) where both the app performing DIRECT SYNC > block IO and the disk firmware implementing CMD_FLUSH can NOT be > modified. In which case the standard block driver behaviour of issuing > a CMD_FLUSH with each write is too restrictive and thus modified using > the patch. Again, what your patch does is to explicitly ignore the data integrity request from the application. While this will usually be way faster, it will also cause data loss. Simply disabling the writeback cache feature of the disk using hdparm will give you much better performance than issueing all the FLUSH command, especially if they are non-queued, but without breaking the gurantee to the application. -- To unsubscribe from this list: send the line "unsubscribe linux-fsdevel" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
