Current implementation of dosfs in libstand reads full File Allocation Table to
the RAM in the initialization code. In the extreme case of FAT32 filesystem,
this structure will take up to 256-1024 Mb of loader memory, depending on the
cluster size.
Proposed patch reduces libstands/dosfs memory requirements to 128 Kb for all
variants of dosfs filesystem. For FAT12 and FAT16 filesystems, File Allocation
Table is cached in full, as before. For FAT32, File Allocation Table is broken
into the equal blocks of 128 Kilobytes (32768 entries), and only current block
is cached.
Because per-filesystem context is now small, global FAT cache (for all
instances of dosfs filesystem) is replaced by local per-instance cache.
Submitted by: Mikhail.Kupchik_gmail.com
Reviewed by: tsoome, allanjude
Differential Revision: https://reviews.freebsd.org/D9547
The block cache implementation in loader has proven to be almost useless, and in worst case even slowing down the disk reads due to insufficient cache size and extra memory copy.
Also the current cache implementation does not cache reads from CDs, or work with zfs built on top of multiple disks.
Instead of an LRU, this code uses a simple hash (O(1) read from cache), and instead of a single global cache, a separate cache per block device.
The cache also implements limited read-ahead to increase performance.
To simplify read ahead management, the read ahead will not wrap over bcache end, so in worst case, single block physical read will be performed to fill the last block in bcache.
Booting from a virtual CD over IPMI:
0ms latency, before: 27 second, after: 7 seconds
60ms latency, before: over 12 minutes, after: under 5 minutes.
Submitted by: Toomas Soome <tsoome@me.com>
Reviewed by: delphij (previous version), emaste (previous version)
Relnotes: yes
Differential Revision: https://reviews.freebsd.org/D4713
directory is like any subdirectory and as such needs to use a real
cluster number. To this end, keep a DE structure for the root in
the DOS_FS structure and populate it accordingly.
While here:
o allow consecutive path separators by skipping them all.
o add missing $FreeBSD$ keyword to dosfs.h.
