A question on write caching

[Robert Mischke]

I'm planning to add a new 120gb drive to my system, but I'm not sure
if I should take a 2Mb or 8Mb cache version. What bothers me is the
following thought: If current hard drives cache writing operations,
isn't the risk of data loss (by power failure etc.) higher with 8mb of
cache? I'm thinking along the lines that if more data is cached, more
time is needed to write it away, so the "window of opportunity" for
data loss is longer. I also seem to remember a windows bug where an
ATX system would power down too fast for disk writes to complete.

So my questions are:

- Is the risk of data loss really higher with a larger cache?

- Is write caching enabled by default on today's drives? (I'm planning
to get either a WD1200BB (2mb) or WD1200JB (8mb).)

- If there's write caching, is it set to write-through or write-back
by default?


Thanks,
Robert

 

[CWatters]

I'm planning to add a new 120gb drive to my system, but I'm not sure
if I should take a 2Mb or 8Mb cache version. What bothers me is the
following thought: If current hard drives cache writing operations,
isn't the risk of data loss (by power failure etc.) higher with 8mb of
cache?

The data doesn't stay in the cache for long. Anyway why worry about 2 vs 8
MByte when you have 512 MByte of main memory? Perhaps you should take some
of that out and send it to me

 

[Ron Reaugh]

I'm planning to add a new 120gb drive to my system, but I'm not sure
if I should take a 2Mb or 8Mb cache version. What bothers me is the
following thought: If current hard drives cache writing operations,
isn't the risk of data loss (by power failure etc.) higher with 8mb of
cache?
Yes.

I'm thinking along the lines that if more data is cached, more
time is needed to write it away, so the "window of opportunity" for
data loss is longer. I also seem to remember a windows bug where an
ATX system would power down too fast for disk writes to complete.
Fixed.

So my questions are:

- Is the risk of data loss really higher with a larger cache?

Probably but that doesn't specify the absolute value of the overall risk
which is generally low in many average situations.

- Is write caching enabled by default on today's drives?
Yes.

(I'm planning
to get either a WD1200BB (2mb) or WD1200JB (8mb).)

- If there's write caching, is it set to write-through or write-back
by default?

Write back. On board writethrough is what happens when onboard caching is
off.

 

[CWatters]

If current hard drives cache writing operations,

Yes.

No. I don't believe it makes any difference. The only "safe" place for data
is on the drive platter itself and a larger cache DOES NOT slow down the
overall process of writing data to the platter - that's fixed by the bit
rate/rpm etc.

 

[Mr. Grinch]

(e-mail address removed) (Robert Mischke) wrote in @news.individual.de:

I'm planning to add a new 120gb drive to my system, but I'm not sure
if I should take a 2Mb or 8Mb cache version. What bothers me is the
following thought: If current hard drives cache writing operations,
isn't the risk of data loss (by power failure etc.) higher with 8mb of
cache? I'm thinking along the lines that if more data is cached, more
time is needed to write it away, so the "window of opportunity" for
data loss is longer. I also seem to remember a windows bug where an
ATX system would power down too fast for disk writes to complete.

So my questions are:

- Is the risk of data loss really higher with a larger cache?

- Is write caching enabled by default on today's drives? (I'm planning
to get either a WD1200BB (2mb) or WD1200JB (8mb).)

Usually it is enabled, or has been on every drive I've bothered to check.
Can't say for your WD drives.

- If there's write caching, is it set to write-through or write-back
by default?

Many drive vendors have tools you can use to view and set the caching info.
That's where I'd look.

I remember Exchange 5.0 used to come with "Exchange Optimizer". On Compaq
Proliant SCSI arrays it would go to each drive and try to turn off the cache.
At least that's what the MS technician for the support call said. Problem
was, on some vendors drives, they ended up changing things other than the
cache setting. I guess MS thought caching was dangerous enough to Exchange
5.0 stores to try and disable it, but I don't think they bother trying to do
that anymore. There are definately other apps out there that recommend
disabling of caches, but in some situations it's just not practical.

 

[CJT]

The data doesn't stay in the cache for long. Anyway why worry about 2 vs 8
MByte when you have 512 MByte of main memory? Perhaps you should take some
of that out and send it to me

I think that misses the point. Losing data in the drive cache
can leave the drive in an inconsistent state. Losing data in
the main memory, even if it's cached disk writes, won't result
in an inconsistent state if you use a logging file system.

 

[CJT]

I'm planning to add a new 120gb drive to my system, but I'm not sure
if I should take a 2Mb or 8Mb cache version. What bothers me is the
following thought: If current hard drives cache writing operations,
isn't the risk of data loss (by power failure etc.) higher with 8mb of
cache? I'm thinking along the lines that if more data is cached, more
time is needed to write it away, so the "window of opportunity" for
data loss is longer. I also seem to remember a windows bug where an
ATX system would power down too fast for disk writes to complete.

So my questions are:

- Is the risk of data loss really higher with a larger cache?

- Is write caching enabled by default on today's drives? (I'm planning
to get either a WD1200BB (2mb) or WD1200JB (8mb).)

- If there's write caching, is it set to write-through or write-back
by default?


Thanks,
Robert

The real problem with some IDE drives is you can't turn the
write cache off (at least I've seen drives where I couldn't --
the software would say it had done so, but when I checked
afterwards the cache was still on).

 

[Robert Mischke]

Thanks for all the answers!