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- Super PC-Kwik: Still the Safest, Fastest Disk Cache
-
- PC-Kwik Corporation has always recognized that reliability is the most
- important issue in designing a disk cache. There are two options that
- can make a disk cache faster, but can also increase the risk of data
- loss: time-delayed writes and request sorting. Super PC-Kwik, our disk
- cache, offers both of these options, but not as a default configuration.
- We strongly feel that a disk cache's default behavior should not increase
- the risk of substantial data loss.
-
- Time-delayed writes and request sorting are, however, automatically
- activated by SMARTDrive, the Microsoft disk cache that's bundled with
- DOS 6 and Window 3.1, as well as by most other commercially marketed caches.
- Although Microsoft has sold millions of copies of DOS 6.0, we found it
- interesting when reports of system failures, crashes, irretrievable data loss,
- etc. were being linked to DOS 6.0. And we found it particularly interesting
- when InfoWorld started attributing some of these problems specifically to
- SMARTDrive.
-
- "Most of the problems eliminated were associated with something DOS 6.0
- does that 5.0 didn't: automatic installation of the SMARTDrive disk cache
- without notifying the user."
- InfoWorld, May 10, 1993
-
- "I strongly urge you to stop using SMARTDrive. I believe that this is the
- source of the 'after a few weeks' DoubleSpace corruption problem."
- Steve Gibson, InfoWorld, May 10, 1993
-
- Time-delayed writes and request sorting pose substantial risks:
-
- 1. Time-delayed writes. Users of PCs normally assume that when
- DOS prompt is displayed and none of their disk lights is on, any disk
- activity is complete. Many users will further assume that it is
- therefore safe to turn their computer off or to reboot. When a disk cache
- has time-delayed writes enabled, as is SMARTDrive's and other disk
- caches' default setting, this is not a safe assumption. In addition, there
- is a chance of a power outage or system hang happening before the data
- is safely written to the disk.
-
- The InfoWorld test lab encountered problems with SMARTDrive due to
- time-delayed writes. "Our cold reboot (equivalent to turning the power off)
- before SMARTDrive's cache had finished flushing was pilot error. But
- SMARTDrive default to "lazy writes" and its immediate return of the DOS
- prompt is a data loss waiting to happen for many users. Although the
- performance gain from delayed writes is substantial, the cache software that
- competes with SMARTDrive can be configured so the user is not returned to the
- DOS prompt before the cache is fully flushed. SMARTDrive has no such
- configuration option." (Infoworld, May 3, 1993)
-
- Not only is Super PC-Kwik fully configurable as InfoWorld reported,
- but Super PC-Kwik automatically synchronizes with the DOS prompt and does
- not use time-delayed writes unless configured to do otherwise. The user
- does not have to worry about the cache not being fully flushed when
- the DOS prompt appears. Once the DOS prompt appears, all disk activity is
- complete; the user is free to turn the computer off or reboot without the
- risk of data loss or corruption.
-
- 2. Request sorting. This feature involves changing the time order
- of writing sectors to the disk so sectors can be written in physical order.
- Assuming that all writes are complete, there is no functional difference
- between FIFO (first in, first out) processing and request sorting of writes
- because all the data ends up on the disk in the originally intended locations.
- But, if a hardware or software failure occurs during disk writing, the
- ability to recover from the failure may be compromised. A well-designed
- database (or other) program takes great care in the order that its index
- and data records are written. This allows the program to recover from a
- crash by eliminating questionable records, without compromising the
- integrity of the database as a whole. When a disk cache changes the time
- order of writes, the assumptions made by the application are no longer true
- and may prevent the application program from recovering from a crash.
-
- The DOS FAT and directory structures can be thought of as a special
- type of database. When DOS creates a file, it carefully postpones pointing
- the new directory entry at the newly allocated disk clusters until the FAT
- has been updated with the allocated clusters. If a disk cache is used that
- changes the order of events and a system crash occurs, the directory entry
- may point to disk clusters that are marked as unallocated in the FAT. If
- the system continues to be used, these same clusters will be allocated to
- another file, and cross-linked files will result. Cross-linked files are
- among the most difficult problems to correct because two files share the
- same space on the disk. One or both of the files will be corrupted with
- no reliable technique for recovering the lost data.
-
- Unlike SMARTDrive and other third party caches, Super PC-Kwik's standard
- mode of operation is to immediately begin processing all write requests on a
- strict FIFO basis. Users who are knowledgeable about time-delayed writes and
- request sorting can enable them and obtain additional performance, but only if
- they choose to take that risk.
-
- Super PC-Kwik does not risk users' data with time-delayed writes and request
- sorting in its default mode. Super PC-Kwik has always been reliable, safe,
- and fast -- automatically.