Storage Technologies Learning Objectives: –Renew acquaintance with disk and file system characteristics –Describe operational limitations of conventional disk usage –Describe how multiple disks, mirroring and striping overcome these difficulties –Appreciate the complexity of these multiple schemes COMP

Storage Technologies Grand Plan: 1.Review/revision, Disks, Filesystems, Limitations and Solutions 2.RAID – build server filestore from PC parts 3.Logical Volume Management, Storage Area Networking, Next-Generation Filesystems (Google, Sun) COMP

Review – Disk Storage Capacity? Power Consumption? Rotation Speed? Seek? Time? Search? Time? Transfer? Rate? Interface type? Max bandwidth? Price? COMP

Review – File Systems Naming Service: Files, Directories, Links Storage Service: Vector of Bytes, owners, permissions, existence control Data and Metadata Space Allocation (file and free-space): Contiguous, Linked (FAT), Indexed Recovery – chkdsk, fsck COMP

Problems With Disks They are (were?) too small They sometimes fail They are too slow They are in the wrong place –(think system virtualization) COMP

Disks Too Small? Use multiple disks How should they be named? –F:/one/two, G:/three/four –/Volumes/first, /Volumes/second –/var/huge-file-store??? Cue: Rawsthornes story about a 2.4Tb file COMP

Disks Too Unreliable? Failure modes: total, partial Use two (or more…) – Mirroring Write to both, read from either If one fails, carry on using other Recreate copy (slowly…) Refinement: read from nearest – accelerate reads COMP

Disks Too Slow? Slow because of: –Transfer rate? –Search time? –Seek time? To minimise seek time, maximize data available with no seek stripe file system across multiple disks COMP

Disk Striping (increases transfer rate) (increases capacity) (decreases reliability) COMP File System Unit 0 File System Unit 1 File System Unit 2 File System Unit 3

Problems With Disks They are (were?) too small –Fixed (multiple disks), fixed (striping) They sometimes fail –Fixed (mirroring), worsened (multiple/striping) They are too slow –Fixed (mirroring/striping) They are in the wrong place (think system virtualization) COMP

Is There A Better Way? Yes, but… see next lecture! COMP