How to determine the used size of device associated's buffer

Question

when mounting a device without the "sync" option, e. g. by invoking the following:

mount -o async /dev/sdc1 /mnt

a buffer is associated with a device to optimize (speed) read/write operations. Is there a way to determine the size of this buffer? Another question that comes into my mind is, if it's possible to find out how much of it is used currently. This can be interesting to determine the time it would take to "sync" or "umount" slow devices, such as flash-based media.

Thanks in advance for your answers, Rainer

Answer 1

There is no fixed size. All systems I'm familiar with (I'm sure about Linux, and I think all unices and even Windows work that way) use as much memory as they can for the disk caches: the read cache size is only limited by the available RAM and the need to keep process memory in RAM too.

The vmstat or top command might print some memory usage statistics. On Linux, free is more readable. It shows how much memory is devoted to the disk caches vs. process memory.

The write buffers also don't have a fixed limit; the decision of when to flush them is based on time and disk load more than size.