SJS wrote:
Should an application call fsync(), the expected result (blocking
until the data is written to physical media) will not happen.
...and that gets me annoyed.
What's wrong with respecting fsync()? This is betraying the user's
trust in the system, really.
Not really. The user knows he just spent a ton of money on one of Violin
Memory's terabyte RAM devices or that they spent somewhat less on a
ton of RAM and then set up Ramback so they know exactly what they are
getting into. Plus they want the apps to run fast and don't want to have
to go through the code and strip out fsync's.
When I was at MP3.com we bought these big Storagetek disk arrays (our
first one was 1.6T in 90 18G Fibrechannel disks yielding around 1T of
usable space after RAID 5 and hot spares etc) and they had two "storage
processors" which were PC-like motherboards with Alpha processors, PCI
bus, DRAM, etc. with something like 512M of cache in each one. And they
would return from fsync and cache the data in RAM before having written
to the disk. They had batteries built into the chassis which would
maintain this RAM and power a special hard disk to write the RAM
contents out to in case of power failure. Overall a pretty similar setup
as the RAMback. Not nearly so much RAM but failure of the system could
ruin your data. And these were very popular units deployed in countless
datacenters in both private businesses and government facilities all
over the world. I've never heard of a single problem resulting from the
way they handle fsync or failing to get the memory written to disk. This
is pretty common in the storage industry and a well accepted practice.
You just need to believe in your battery, Linux and the hardware it
runs on. Which of these do you mistrust?
Duh. All of 'em.
I think a system can be built which would work quite well and which I
would trust. Others have done it many times before as I note above. It
wouldn't be Wal-mart hardware. And it wouldn't be running X or a bunch
of other extraneous stuff. And yes, I would keep close tabs on the UPS
and wouldn't buy the cheap home-office unit from Best Buy.
I have six UPSes at home. All of them have failed at one time or
another. Batteries are consumable items, and who goes around replacing
a UPS battery at home once a year "just in case"?
This isn't for a home application. People who use this kind of
technology and really need to rely on it do indeed test and change out
batteries on a regular basis.
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