> From: joystick <[email protected]> > > I don't really understand this disk cache thing. > Suppose a disk with write cache enabled of writeback type: Linux > receives a write completed notification (a message from the disk) when > the data has reached the cache of the disk. Correct? At that point it is > not considered an in-flight I/O anymore. Correct? > So what happens when the disk tries to write it to the platter and > discovers that there is a media error on that sector? (suppose > relocation does not happen ; maybe sectors exhausted) > Does Linux receive the write error upon the next flush it issues? So the > error is related to the flush? And what happens if Linux never issues > such flush?
Thinking about this some more, there's no particular reason for the kernel to use write-back caching in the disk, since the kernel itself maintains a write-back cache. So the kernel can be quite patient about when the disk writes the block to the platters. Given all the complexities of coordinating with a disk that is doing write-back caching, it seems like it would be easier for the kernel to tell the disk to write-through, that is, don't notify the kernel that the write is done until the data is actually on the platter. Most of the time, the kernel can be patient about writing; the only time the kernel has to care is when an application forces synchronization on the disk I/O subsystem. (I vaguely recall this is done only on close() or fsync().) And when that happens, the kernel has to know the data is on the platter. Dale -- To unsubscribe from this list: send the line "unsubscribe linux-scsi" in the body of a message to [email protected] More majordomo info at http://vger.kernel.org/majordomo-info.html
