On Sat, 16 Nov 2002, Joseph Kruskal wrote: > On 11/1/02 3:47 AM, William H. Magill at [EMAIL PROTECTED] wrote: > > > ... journaled file system ... > > What is a journaled file system?
This has been answered already, but just to expand a bit: one very nice feature of the late lamented BeOS was it's filesystem, BFS. Among other neat properties was the fact that it was journalled, meaning that before performing operations on the disc itself, the system would make a note of what it was about to attempt to a special log area first. The benefit of this design is that if anything goes wrong later on, you can quickly roll back to a stable system state. Along similar lines, most modern transactional database systems do something similar -- for example, they'll record your SQL queries to a transaction log, then attempt to do those queries. That way, if a query fails -- you get a disk full error or power outage, for example -- you can leave the database unharmed. (Going back a bit further, IIRC accountants record day to day activity to their own transaction log -- the general ledger -- before using that information to prepare various financial documents. If the documents are found to have errors (arithmatic errors, Enron fraud, etc) then if nothing else auditors can still go back to the original ledgers for comparison. Anyway, the guy that designed BFS, Dominic Giampalo, previously worked for SGI, where he gave Irix supercomputer systems their own journaling layer. For BFS, this allowed average PCs to boot in 15 seconds instead of 5 minutes, but for the big SGI machines it was the difference between 5 minutes and 5 hours, so really it was a critical improvement to a legacy system. Now, Dominic has come to Apple, where he's repeated that trick for HFS+. As others said, this will generally slow down your system a little bit on an ongoing basis, because when writing to disc you have to do a new step of also writing to the system journal first. On the other hand, this should make boot times quicker, particularly for large drives & after system crashes, and it should give much better data integrity as well. This now brings OSX one big step closer to where BeOS was five years ago, and where Irix was a decade ago ;) -- Chris Devers [EMAIL PROTECTED]
