On 2019-07-01 10:01, Warren Young wrote:
On Jul 1, 2019, at 8:26 AM, Valeri Galtsev <galt...@kicp.uchicago.edu> wrote:
RAID function, which boils down to simple, short, easy to debug well program.
I didn't intend to start software vs hardware RAID flame war when I
joined somebody's else opinion.
Now, commenting with all due respect to famous person who Warren Young
definitely is.
RAID firmware will be harder to debug than Linux software RAID, if only because
of easier-to-use tools.
I myself debug neither firmware (or "microcode", speaking the language
as it was some 30 years ago), not Linux kernel. In both cases it is
someone else who does the debugging.
You are speaking as the person who routinely debugs Linux components. I
still have to stress, that in debugging RAID card firmware one has small
program which this firmware is.
In the case of debugging EVERYTHING that affects reliability of software
RAID, on has to debug the following:
1. Linux kernel itself, which is huge;
2. _all_ the drivers that are loaded when system runs. Some of the
drivers on one's system may be binary only, like NVIDIA video card
drives. So, even for those who like Warren can debug all code, these
still are not accessible.
All of the above can potentially panic kernel (as they all run in kernel
context), so they all affect reliability of software RAID, not only the
chunk of software doing software RAID function.
Furthermore, MD RAID only had to be debugged once, rather that once per
company-and-product line as with hardware RAID.
Alas, MD RAID itself not the only thing that affects reliability of
software RAID. Panicking kernel has grave effects on software RAID, so
anything that can panic kernel had also to be debugged same thoroughly.
And it always have to be redone once changed to kernel or drivers are
introduced.
I hope you’re not assuming that hardware RAID has no bugs. It’s basically a
dedicated CPU running dedicated software that’s difficult to upgrade.
That's true, it is dedicated CPU running dedicated program, and it keeps
doing it even if the operating system crashed. Yes, hardware itself can
be unreliable. But in case of RAID card it is only the card itself.
Failure rate of which in my racks is much smaller that overall failure
rate of everything. In case of kernel panic, any piece of hardware
inside computer in some mode of failure can cause it.
One more thing: apart from hardware RAID "firmware" program being small
and logically simple, there is one more factor: it usually runs on RISC
architecture CPU, and introduce bugs programming for RISC architecture
IMHO is more difficult that when programming for i386 and amd64
architectures. Just my humble opinion I carry since the time I was
programming.
if kernel (big and buggy code) is panicked, current RAID operation will never
be finished which leaves the mess.
When was the last time you had a kernel panic? And of those times, when was
the last time it happened because of something other than a hardware or driver
fault? If it wasn’t for all this hardware doing strange things, the kernel
would be a lot more stable. :)
Yes, I half expected that. When did we last have kernel crash, and who
of us is unable to choose reliable hardware, and unable to insist that
our institution pays mere 5-10% higher price for reliable box than they
would for junk hardware? Indeed, we all run reliable boxes, and I am
retiring still reliably working machines of age 10-13 years...
However, I would rather suggest to compare not absolute probabilities,
which, exactly as you said, are infinitesimal. But with relative
probabilities, I still will go with hardware RAID.
You seem to be saying that hardware RAID can’t lose data. You’re ignoring the
RAID 5 write hole:
https://en.wikipedia.org/wiki/RAID#WRITE-HOLE
Neither of our RAID cards runs without battery backup.
If you then bring up battery backups, now you’re adding cost to the system.
And then some ~3-5 years later, downtime to swap the battery, and more
downtime. And all of that just to work around the RAID write hole.
You are absolutely right about system with hardware RAID being more
expensive than that with software RAID. I would say, for "small scale
big storage" boxes (i.e. NOT distributed file systems), hardware RAID
adds about 5-7% of cost in our case. Now, with hardware RAID all
maintenance (what one needs to do in case of single failed drive
replacement routine) takes about 1/10 of a time necessary do deal with
similar failure in case of software RAID. I deal with both, as it
historically happened, so this is my own observation. Maybe software
RAID boxes I have to deal with are too messy (imagine almost two dozens
of software RAIDs 12-16 drives each on one machine; even bios runs out
of numbers in attempt to enumerate all drives...) No, I am not taking
the blame for building box like that ;-)
All in all, simpler way of routinely dealing with hardware RAID saves
human time involved, and in a long run quite likely is money saving
(think of salaries, benefits etc.), though it looks more expensive at
the moment of hardware purchase.
Copy-on-write filesystems like ZFS and btrfs avoid the write hole entirely, so
that the system can crash at any point, and the filesystem is always consistent.
Well, yes, zfs is wholly different dimension, and neither of my comments
meant to be made in presence of zfs ;-)
I guess, our discussions (this, I'm sure, in not the first one) leaves
each of us with our own opinions unchanged, so this will be my last set
of comments on this thread; whoever reads it will be able to use one's
own brain and arrive at one's own conclusions.
With a hope this helps somebody,
Valeri
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Valeri Galtsev
Sr System Administrator
Department of Astronomy and Astrophysics
Kavli Institute for Cosmological Physics
University of Chicago
Phone: 773-702-4247
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