[Filestore] Provide client with correct blocksize

[debnatkh]

Using stat, a client can get a blocksize, which, according to man(2), is defined as

The st_blksize field gives the "preferred" blocksize for efficient file system I/O. (Writing to a file in smaller chunks may cause an inefficient read-modify-rewrite.)

Right now this field is independent of the actual blocksize (in terms of filestore) and equals 4KiB.

For example, this results in cat breaking one 1MiB request into smaller requests of size max(128KiB, blocksize) = 128KiB. This would be more efficient if we provided a client with blocksize of bs*64 (256KiB in case of the default bs).

[debnatkh]

Successfully deployed PreferredBlockSizeMultiplier: 64. It had no effect on ior performance when using it without O_DIRECT. However, it indeed decreased number of read(2) syscalls while using cat on large files, as expected.

Also it had some benefitial impact on the dd/cp performance:

PreferredBlockSizeMultiplier	1	64
dd if=/dev/urandom of=mnt/testfile bs=1G count=1	57s	38s
cp (1 GiB file) mnt/cp (1 GiB file) mnt/	1m45.804s	0m44.533s