Re: Uhh...Informix on block device?!
Mr Cheuk Kong Lo wrote: | |Dear Gurus, | |I'm just taking up a sysadm/DBA on a new job. Just found that our |Informix is using raw chunks but these chunks point to the block device |under /dev. (e.g., /dev/lv00 as opposed to /dev/rlv00). | |This is _extremely_ weird to me, as I know we should use /dev/rlv00, |but Informix is still happy about it. I'm concerned about the /dev/lv00 |is buffered I/O device and something worst could happen. On the other |hand, a simple "dd" tests shows (not surprisingly) the block device are |faster than raw devices...so... | |Any input to help me to talk out with management... David Williams responded: |With a buffered I/O device Online cannot guarantee that data written |to the device will actually get written to disk. |Hence reliability decreases. Yup, that should be the primary concern: reliability. Because as soon as the engine thinks it is out there on disk, it feels free to eliminate some in-memory structures. Another gadfly in the ointment: I was called to consult at a client who was experiencing frequent lock-ups of the entire system. Not just occasional but frequent, system-wide (not only Informix stuff) just hanging for a half-minute at a time. AIX has the coolest graphic monitoring tools, so I was able to get some handle on when the lockups were happening and (surprise!) they were coincident with checkpoints. The problem turned out that the entire system was built on raw BLOCK devices. As David pointed out, these are cached. Well, at checkpoint time, the engine was choking the Unix disk cache with ~ 10,000 buffer writes - 40mb or so - in a Unix buffer pool of about 5mb. The quickie solution was to increase the Unix cache. The real solution was, of course, to move the data to raw character devices. -- -- Jake (In pursuit of undomesticated aquatic avians) +-----------------------------------------------------------+ | Impeccable Logic: A thought process which successfully | | resists chicken bites | +-----------------------------------------------------------+