Warren Young wrote on 6/28/2019 6:53 PM:
On Jun 28, 2019, at 8:46 AM, Blake Hudson <blake@xxxxxxxx> wrote:
Linux software RAID…has only decreased availability for me. This has been due to a combination of hardware and software issues that are are generally handled well by HW RAID controllers, but are often handled poorly or unpredictably by desktop oriented hardware and Linux software.
Would you care to be more specific? I have little experience with software RAID, other than ZFS, so I don’t know what these “issues” might be.
I've never used ZFS, as its Linux support has been historically poor. My
comments are limited to mdadm. I've experienced three faults when using
Linux software raid (mdadm) on RH/RHEL/CentOS and I believe all of them
resulted in more downtime than would have been experienced without the RAID:
1) A single drive failure in a RAID4 or 5 array (desktop IDE)
caused the entire system to stop responding. The result was a degraded
(from the dead drive) and dirty (from the crash) array that could not be
rebuilt (either of the former conditions would have been fine, but not
both due to buggy Linux software).
2) A single drive failure in a RAID1 array (Supermicro SCSI) caused
the system to be unbootable. We had to update the BIOS to boot from the
working drive and possibly grub had to be repaired or reinstalled as I
recall (it's been a long time).
3) A single drive failure in a RAID 4 or 5 array (desktop IDE) was
not clearly identified and required a bit of troubleshooting to pinpoint
which drive had failed.
Unfortunately, I've never had an experience where a drive just failed
cleanly and was marked bad by Linux software RAID and could then be
replaced without fanfare. This is in contrast to my HW raid experiences
where a single drive failure is almost always handled in a reliable and
predictable manner with zero downtime. Your points about having to use a
clunky BIOS setup or CLI tools may be true for some controllers, as are
your points about needing to maintain a spare of your RAID controller,
ongoing driver support, etc. I've found the LSI brand cards have good
Linux driver support, CLI tools, an easy to navigate BIOS, and are
backwards compatible with RAID sets taken from older cards so I have no
problem recommending them. LSI cards, by default, also regularly test
all drives to predict failures (avoiding rebuild errors or double failures).
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