The Lies Disks and Their Drivers Tell 192
davecb writes "Pity the poor filesystem designer: they just want to know when their data is safe, but the disks and drivers try so hard to make I/O 'easy' that it ends up being stupidly hard. Marshall Kirk McKusick writes about the difficulties in making the systems work nicely together: 'In the real world, many of the drives targeted to the desktop market do not implement the NCQ specification. To ensure reliability, the system must either disable the write cache on the disk or issue a cache-flush request after every metadata update, log update (for journaling file systems), or fsync system call. Both of these techniques lead to noticeable performance degradation, so they are often disabled, putting file systems at risk if the power fails. Systems for which both speed and reliability are important should not use ATA disks. Rather, they should use drives that implement Fibre Channel, SCSI, or SATA with support for NCQ.'"
Re:almost clicked the link... (Score:4, Interesting)
I still bet those drives if you pull power on them will lose the data in their onboard caches.
Which means they are lying about fsync.
Re:Get Hardware RAID (Score:4, Interesting)
The only real advantage to "Hardware RAID" is the battery backed cache. Hardware RAID comes with the disadvantage of a whole other operating system "firmware" with its own bugs and often proprietary disk layout. Parity calculations are nothing for current CPUs, so the onboard processor is not so useful. Advanced filesystems such as ZFS or BTRFS need direct access to the disks. I'd like to see drives and/or controllers with battery backed cache. Until then, I rely on my UPS.
Linus's Input on Write Cache (Score:4, Interesting)
I think this is quite interesting.
http://yarchive.net/comp/linux/drive_caches.html [yarchive.net]
While I've often gotten the impression that the write cache opens up a large "write hole", Linus says that data is cached only for milliseconds, not held in the cache for several seconds. Still, I'd like to see battery backed caches in regular drives and/or controllers.
Would be nice to hear from some drive firmware writers.