> Does the availability of SSDs affect the desirability of quantity of disks > vs. size, Christof? >
SSD is still expensive, and yes, you would still want multiple, because rebuilding a smaller disk takes less time than rebuilding a larger disk and thus leaves your RAID vulnerable for a shorter period. It's less important than with spindles, though, since SSD have IOPS a hundred times higher. The biggest issue with SSD in servers is that server SSD are very different from desktop SSD. SSD don't work like memory where you can simply change a byte. They operate on pages just like a disk. Unlike a disk, however, you can't just overwrite an existing page, the page has to be all zero before you can set bits. Therefore replacing an existing record requires the disk to first blank the page, then write new content. Since that operation costs time, the SSD usually moves sectors around. If you change a sector, the SSD is looking for an empty page and puts the new sector in that page. Then it marks the existing page pending and puts it into a queue. When the disk is idle, the drive purges old pages. Because of this an SSD drive gets exponentially slower the fuller it gets. All SSD drives have more memory than they report as disk space. A desktop SSD has only a bit more, a server SSD usually has twice or three times the memory, so it always finds a blank page even with heavy load. Thats where TRIM support is important, on the SSD side and the software side (ESXi, for instance, does not support TRIM). TRIM means that the operating system reports to the disk that a sector may contain data, but that the file system considers this block to be empty and will overwrite it when demand is there. That happens, for instance, when you delete a file. The content is still there, the file is only removed from the index. With TRIM all sectors are additionally reported as empty. This gives the drive the opportunity to blank those pages when its idle and increasing the number of empty blocks. SSD often work on 4K pages. You need to know this, because otherwise you might just change 1024 bytes (two sectors) that are in different pages, causing 8 KB to be read, purged and written. -- Christof --- StripMime Report -- processed MIME parts --- multipart/alternative text/plain (text body -- kept) text/html --- _______________________________________________ Post Messages to: [email protected] Subscription Maintenance: http://mail.leafe.com/mailman/listinfo/profox OT-free version of this list: http://mail.leafe.com/mailman/listinfo/profoxtech Searchable Archive: http://leafe.com/archives/search/profox This message: http://leafe.com/archives/byMID/profox/CAL4QJhjKF=tr4z1ho_eoaizwmetcsg2b+bofvgbkixqbdcu...@mail.gmail.com ** All postings, unless explicitly stated otherwise, are the opinions of the author, and do not constitute legal or medical advice. This statement is added to the messages for those lawyers who are too stupid to see the obvious.
