You said: "Have a quick look at http://lion.drogon.net/mrtg/diskIO.html"; Are you crazy! Quick look, my screen is 1600X1200. Your quick look is over twice that size! :) What is all the red? Oh, it's eye strain! :) Why do your graphs read right to left? It makes my head hurt! Well, I am surprised. I have read somewhere that about a 10 to 1 ratio is common. That's 10 reads per 1 write! Maybe you got your ins and outs reversed! If you data set is small enough to fit in the buffer cache, then you may be correct. I have worked on systems with a database of well over 1T bytes. The system had about 10T bytes total. No way for all that to fit in the buffer cache! But, I don't have any data to deny what you say. Here is my home system using iostat: # iostat -d Linux 2.4.28 (watkins-home.com) 01/10/2005 Device: tps Blk_read/s Blk_wrtn/s Blk_read Blk_wrtn dev8-0 1.77 209.57 10.08 38729760 1862232 dev8-1 1.78 210.75 10.08 38948102 1862232 dev8-2 10.13 522.27 36.50 96518688 6745616 dev8-3 10.52 524.71 37.94 96970042 7011712 dev8-4 10.55 525.05 38.00 97032904 7021816 dev8-5 10.60 524.90 37.99 97004392 7021080 dev8-6 10.59 524.86 38.17 96997424 7054816 dev8-7 10.56 524.89 38.23 97002160 7064552 dev8-8 10.54 524.73 38.25 96973096 7068552 dev8-9 10.54 524.55 37.89 96940336 7001736 dev8-10 10.15 522.54 36.74 96568080 6789584 dev8-11 10.18 522.51 36.52 96562208 6749696 dev8-12 10.21 522.60 36.74 96578592 6790600 dev8-13 10.22 522.67 37.10 96592848 6856800 dev8-14 10.17 522.46 36.95 96552728 6828544 dev8-15 10.19 522.30 36.70 96523856 6782136 The first 2 are my OS disks (RAID1). The others are my 14 disk RAID5. That's about 13 to 1 on my RAID5 array. But I would not claim my system is typical. It is a home system, not a database server or fancy web server. Mostly just a samba server. My email server is a different box. Oops, I just checked my email server, it has 64 meg of RAM and only does email all the time. Device: tps Blk_read/s Blk_wrtn/s Blk_read Blk_wrtn dev3-0 1.32 2.10 19.72 7284164 68380122 That's 1 to 9. I give up! I got to mirror that system some day! Guy -----Original Message----- From: linux-raid-owner@xxxxxxxxxxxxxxx [mailto:linux-raid-owner@xxxxxxxxxxxxxxx] On Behalf Of Gordon Henderson Sent: Monday, January 10, 2005 2:42 AM To: linux-raid@xxxxxxxxxxxxxxx Subject: RE: Spares and partitioning huge disks On Sun, 9 Jan 2005, Guy wrote: > Why is RAID1 preferred over RAID5? > RAID1 is considered faster than RAID5. Most systems tend to read much more > than they write. You'd think that, wouldn't you? However, - I've recently been doing work to graph disk IO by reading /proc/partitions and feeding it into MRTG - what I saw surprised me, although it really shouldn't. Most of the systems I've been graphing over the past few weeks write all the time and rarely read -I'm putting this down to things like log files being written more or less all the time, and the active data set residing in the filesystem/buffer cache more or less all the time. (also ext3 which wants to write all the time too) However, I guess it all depends on what the server is doing - for a workstion it may well be the case that it does more reads. Have a quick look at http://lion.drogon.net/mrtg/diskIO.html This is a moderately busy web server with a couple of dozen virtual web sites and runs a MUD and several majordomo lists. Blue is writes, Green reads. Note periods of heavy read activity just after midnight when it does a backup (over the 'net to another server and it also sucks another server onto the 'archive' partition), and 2am is when it analyses the web log-files. Also note that it's swapping - this has 256MB of RAM and is due for an upgrade, but swap is keeping it all ticking away nicely. The var partition seems to sustain writes at approx. 200-300 sectors/second... Not a fantastic amount, but I found it rather surprising. (I'll put the MRTG code online for anyone who wants it in a few days and let you know) Gordon - To unsubscribe from this list: send the line "unsubscribe linux-raid" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html - To unsubscribe from this list: send the line "unsubscribe linux-raid" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
