fred@mot.UUCP (Fred Christiansen) (01/04/85)
[] anyone have experience with these? do they really help? the sector cachers say you gotta do it this way because of Unix' habit of fragmenting a file all over the place. but, since Unix caches blocks anyway, does sector caching help? my guess is it would if the controllers memory exceeded kernel's buffer cache. the track cachers say their scheme helps for now and will be best once scatter/gather HW memory management is available. the latter seems contrary to the free list, which is fairly integral to the current Unix kernel's view of the file system (no reason why there couldn't be another, I suppose).
guy@rlgvax.UUCP (Guy Harris) (01/06/85)
> the sector cachers say you gotta do it this way because of Unix' habit of > fragmenting a file all over the place.... > the latter seems contrary to the free list, which is fairly integral to > the current Unix kernel's view of the file system (no reason why there > couldn't be another, I suppose). Can you say "4.2BSD"? I thought you could! I suspect several other variants of the UNIX file system have also been done that use a bit map rather than a free list; I suspect most other operating systems use bit maps as well. The only think I can see that a free list buys you is that if you have limited main memory and disks big enough that significant portions of the bit map couldn't be kept in the buffer cache, grabbing the first blcok off the free list rather than making an effort to pick an optimally positioned block would require fewer disk accesses (it would also require less CPU time under any circumstances). Since the days of 256K PDP-11s are behind us, I don't see the free list being worth much anymore... Guy Harris {seismo,ihnp4,allegra}!rlgvax!guy
dan@ciprico.UUCP (01/10/85)
Here at ciprico we make a caching disk controller for the multibus. SMD interface. I don't have any real numbers handy at the moment but caching helps a great deal...approx. +30% performance. And, a lot more depends on how the caching is performed. We did a study here to see how unix really accessed the disk (this means requests handed to the driver). With some intelligent caching I'd think we could increase the performance by at least 50%. By all means look into caching disk controllers....if you find anything interesting I'd be interested in hearing about it. Oh yes...the study was done on System III...512 byte file system blocks. ------- -Dan A. Dickey ihnp4!mmm!ciprico!dan