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CIT 470: Advanced Network and System Administration

CIT 470: Advanced Network and System Administration. Disks. Topics. Disk interfaces Disk components Performance Reliability Partitions RAID Adding a disk Logical volumes Filesystems Storage Management. Volumes. A volume is a chunk of storage as seen by the server. A disk.

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CIT 470: Advanced Network and System Administration

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  1. CIT 470: Advanced Network and System Administration Disks CIT 470: Advanced Network and System Administration

  2. Topics • Disk interfaces • Disk components • Performance • Reliability • Partitions • RAID • Adding a disk • Logical volumes • Filesystems • Storage Management CIT 470: Advanced Network and System Administration

  3. Volumes A volume is a chunk of storage as seen by the server. • A disk. • A partition. • A RAID set. Logical Unit Numbers (LUNs) identify volumes. A volume is formatted with a filesystem. • ext2 and ext3 • ZFS • FAT • NTFS • ISO9660 CIT 470: Advanced Network and System Administration

  4. Disk Interfaces SCSI Standard interface for servers. IDE Standard interface for PCs. Fibre Channel (FC-AL) High bandwidth Can run SCSI or IP iSCSI SCSI over fast (e.g., 10-gigabit) IP network equipment. USB Fast enough for slow devices on PCs. CIT 470: Advanced Network and System Administration

  5. SCSI Small Computer Systems Interface Fast, reliable, expensive. A bus, not a simple PC to device interface. Each device has a target # ranging 0-7 or 0-15. Devices can communicate directly w/o CPU. Many versions Original: SCSI-1 (1979) 5MB/s Current: SCSI-3 (2001) 320MB/s Serial Attached SCSI (SAS) Up to 128 devices Up to 750 MB/s full duplex. CIT 470: Advanced Network and System Administration

  6. IDE Integrated Drive Electronics / AT attachment Slower, less reliable, cheap. Only allows 2 devices per interface. ATAPI standard added removable devices. Many versions Original: IDE / ATA (1984) Current: Ultra-ATA/133 133MB/s Serial ATA Up to 128 devices. 150 MB/s (SATA-1) and 300 MB/s (SATA-2) CIT 470: Advanced Network and System Administration

  7. IDE vs. SCSI SCSI offers better performance/scale Faster bus Faster hard drives (up to 15,000rpm). Lower CPU usage Better handling of multiple requests. Cheaper IDE often best for workstations. Convergence SATA2 and SAS converging on a single std. CIT 470: Advanced Network and System Administration

  8. Hard Drive Components CIT 470: Advanced Network and System Administration

  9. Hard Drive Components Actuator Moves arm across disk to read/write data. Arm has multiple read/write heads (often 2/platter.) Platters Rigid substrate material. Thin coating of magnetic material stores data. Coating type determines areal density: Gbits/in2 Spindle Motor Spins platters from 3600-15,000 rpm. Speed determines disk latency. Cache 8-32MB of cache memory Reliability: write-back vs. write-through CIT 470: Advanced Network and System Administration

  10. Disk Information: hdparm # hdparm -i /dev/hde /dev/hde: Model=WDC WD1200JB-00CRA1, FwRev=17.07W17, SerialNo=WD-WMA8C4533667 Config={ HardSect NotMFM HdSw>15uSec SpinMotCtl Fixed DTR>5Mbs FmtGapReq } RawCHS=16383/16/63, TrkSize=57600, SectSize=600, ECCbytes=40 BuffType=DualPortCache, BuffSize=8192kB, MaxMultSect=16, MultSect=off CurCHS=16383/16/63, CurSects=16514064, LBA=yes, LBAsects=234441648 IORDY=on/off, tPIO={min:120,w/IORDY:120}, tDMA={min:120,rec:120} PIO modes: pio0 pio1 pio2 pio3 pio4 DMA modes: mdma0 mdma1 mdma2 UDMA modes: udma0 udma1 udma2 udma3 udma4 *udma5 AdvancedPM=no WriteCache=enabled Drive conforms to: device does not report version: * signifies the current active mode CIT 470: Advanced Network and System Administration

  11. Disk Performance Seek Time Time to move head to desired track (3-8 ms) Rotational Delay Time until head over desired block (8ms for 7200) Latency Seek Time + Rotational Delay Throughput Data transfer rate (20-100 MB/s) CIT 470: Advanced Network and System Administration

  12. Latency vs. Throughput Which is more important? Depends on the type of load. Sequential access – Throughput Multimedia on a single user PC Random access – Latency Most servers How to improve performance Faster disks (15 krpm vs 7200rpm) Caching (disk, controller, server OS, client OS) More spindles (disks). More disk controllers. CIT 470: Advanced Network and System Administration

  13. Disk Performance: hdparm # hdparm -tT /dev/sda /dev/sda: Timing cached reads: 1954 MB in 2.00 seconds = 977.02 MB/sec Timing buffered disk reads: 268 MB in 3.02 seconds = 88.66 MB/sec CIT 470: Advanced Network and System Administration

  14. Reliability MTBF Average time between failures (>100,000 hours). Real failure curves Early phase: high failure rate from defects. Constant failure rate phase: MTBF valid. Wearout phase: high failure rate from wear. Failures more likely on traumatic events. Power on/off. Systems often wear out before MTBF. CIT 470: Advanced Network and System Administration

  15. Partitions and the MBR 4 primary partitions. One can be used as an extended partition, which is a link to an Extended boot record on the 1st sector of that partition. Each logical partition is described by its own EBR, which links to the next EBR. CIT 470: Advanced Network and System Administration

  16. Extended Partitions and EBRs There is only one extended partition. • It is one of the primary partitions. • It contains one or more logical partitions. • It should contain all disk space not used by the other primary partitions. EBRs contain two entries. • The first entry describes a logical partition. • The second entry points to the next EBR if there are more logical partitions after the current one. CIT 470: Advanced Network and System Administration

  17. Why Partition? • Separate OS from user files, to allow user backups + OS upgrades w/o problems. • Have a faster swap area for virtual memory. • Improve performance by keeping filesystem tables small and keeping frequently used together files close together on the disk. • Limit the effect of disk full issues, often caused by log or cache files. • Multi-boot systems with multiple OSes. CIT 470: Advanced Network and System Administration

  18. RAID Redundant Array of Independent Disks Combine physical disks into single logical unit. Can be implemented in hardware or software. Hardware RAID controllers: Caching Automate rebuilding of arrays Advantages Capacity Reliability Throughput CIT 470: Advanced Network and System Administration

  19. RAID Levels CIT 470: Advanced Network and System Administration

  20. Striping • Distribute data across multiple disks. • Improve I/O by accessing disks in parallel. • Independent requests can be serviced in parallel by separate disks. • Single multi-block requests can be serviced by multiple disks. • Performance vs. reliability • Performance increases with # disks. • Reliability decreases with # disks. CIT 470: Advanced Network and System Administration

  21. Parity Store extra bit with each chunk of data. Odd parity • add 0 if # of 1s is odd • add 1 if # of 1s is even Even parity • add 0 if # of 1s is even • add 1 if # of 1s is odd CIT 470: Advanced Network and System Administration

  22. Error Detection with Parity Even: every byte must have even # of 1s. What if you read a byte with an odd # of 1s? • It’s an error. • An odd # of bits were flipped. What if you read a byte with an even # of 1s? • It may be correct. • It may be an error where an even # of bits are bad. CIT 470: Advanced Network and System Administration

  23. Error Correction XOR each block to get parity information. XOR with parity block to retrieve missing block on bad drive. CIT 470: Advanced Network and System Administration

  24. RAID 0: Striping, no Parity Performance Throughput = n * disk speed Reliability • Lower reliability. • If one disk lost, entire set is lost. • MTBF = (avg MTBF)/# disks Capacity n * disk size CIT 470: Advanced Network and System Administration

  25. RAID 1: Disk Mirroring Performance • Reads are faster since read operations will return after first read is complete. • Writes are slower because write operations return after second write is complete. Reliability • System continues to work after one disk dies. • Doesn’t protect against disk or controller failure that corrupts data instead of killing disk. • Doesn’t protect against human or software error. Capacity • n/2 * disk size CIT 470: Advanced Network and System Administration

  26. RAID 3: Striping + Dedicated Parity Reliability Survive failure of any 1 disk. Performance • Striping increases performance, but • Parity disk must be accessed on every write. • Parity calculation decreases write performance. • Good for sequential reads (large graphics + video files.) Capacity (n-1) * disk size CIT 470: Advanced Network and System Administration

  27. RAID 4: Stripe + Block Parity Disk • Identical to RAID 3 except uses block striping instead of byte striping. CIT 470: Advanced Network and System Administration

  28. RAID 5: Stripe + Distributed Parity Reliability Survive failure of any 1 disk. Performance • Fast reads (RAID 0), but slow writes. • Like RAID 4 but without bottleneck of a single parity disk. • Still have to read blocks + write parity block if alter any data blocks. Capacity (n-1) * disk size CIT 470: Advanced Network and System Administration

  29. RAID 6: Striped w/ Dual Parity • Like RAID 5 but with two parity blocks. • Can survive failure of two drives at once. CIT 470: Advanced Network and System Administration

  30. Nested RAID Levels Many RAID systems can use both • Physical drives. • RAID sets. as RAID volumes: • Allows admins to combine advantages of levels. • Nested levels named by combination of levels, e.g. RAID 01 or RAID 0+1 CIT 470: Advanced Network and System Administration

  31. RAID 01 (0+1) Mirror of stripes. • If disk fails in RAID 0 array, can be tolerated by using disk from other RAID 0. • Cannot tolerate 2 disk failures unless both from same stripe. CIT 470: Advanced Network and System Administration

  32. RAID 10 (1+0) Stripe of mirrors. • Can tolerate all but one drive can failing from each RAID 1 set. • Uses more disk space than RAID 5 but provides higher performance. • Highest capacity, performance, and cost. CIT 470: Advanced Network and System Administration

  33. RAID 51 (5+1) Mirror of RAID 5s Capacity = (n/2-1) * disk size Min disks: 6 CIT 470: Advanced Network and System Administration

  34. RAID Failures RAID sets work after single disk failure • Except RAID 0 • Operate in degraded mode RAID set rebuilds after bad disk replaced • Can take hours to rebuild parity/mirror data. • Some hardware allows hot swapping, so server doesn’t have to be rebooted to replace disk. • Some hardware supports a hot spare disk that will be used immediately on disk failure for rebuild. CIT 470: Advanced Network and System Administration

  35. You still need backups Human and software errors • RAID won’t protect you from rm –rf / or copying over the wrong file. System crash • Crashes can interrupt write operations, leading to situation where data is updated but parity not. Correlated disk failures • Accidents (power failures, dropping the machine) can impact all disks at once. • Disks bought at same time often fail at same time. Hardware data corruption • If a disk controller writes bad data, all disks will have the bad data. CIT 470: Advanced Network and System Administration

  36. Logical Volumes What are logical volumes? Appear to user as a physical volume. But can span multiple partitions and/or disks. Why logical volumes? Aggregate disks for performance/reliability. Grow and shrink logical volumes on the fly. Move logical volumes btw physical devices. Replace volumes w/o interrupting service. CIT 470: Advanced Network and System Administration

  37. LVM CIT 470: Advanced Network and System Administration

  38. LVM Components Logical Volume Group (LVG) Set of physical volumes (partitions or disks.) May be divided into logical volumes (LVs.) LVs made up of fixed sized logical extents Each LE is 4MB. Physical extents are the same size. CIT 470: Advanced Network and System Administration

  39. Mapping Modes Linear Mapping LVs assigned to continguous areas of PV space. Striped Mapping LEs interleaved across PVs to improve performance. CIT 470: Advanced Network and System Administration

  40. Setting up a LVG and LV • Initialize physical volumes pvcreate /dev/hda1 pvcreate /dev/hdb1 • Initialize a volume group vgcreate nku_proj /dev/hda1 /dev/hdb1 Use vgextend to add more PVs later. • Create logical volumes lvcreate -n nku1 --size 100G nku_proj1 • Create filesystem mkfs –v –t ext3 /dev/nku_proj/nku1 CIT 470: Advanced Network and System Administration

  41. Extending a LV Set absolute size lvextend –L120G /dev/nku_proj/nku1 Or set relative size lvextend –L+20G /dev/nku_proj/nku1 Expand the filesystem without unmounting ext2online –v /dev/nku_proj/nku1 Check size df –k CIT 470: Advanced Network and System Administration

  42. Adding a Disk Install new hardware Verify disk recognized by BIOS. Boot Verify device exists in /dev Partition fdisk /dev/sdb Create filesystem mkfs –v –t ext3 /dev/sdb1 Add entry to /etc/fstab /dev/sdb1 /proj ext3 defaults 0 2 mount -a CIT 470: Advanced Network and System Administration

  43. Filesystems ext3fs Current common Linux filesystem. Journaling eliminates need for regular fscking. ext2fs Old Linux non-fragmenting fast filesystem. CIT 470: Advanced Network and System Administration

  44. inode • File consists of inode + data blocks. • inodes are static: • table has fixed # inodes • size: 128 bytes • inode contains • UID of owner • GID of group • Permissions • Timestamps • Reference count • Block pointers CIT 470: Advanced Network and System Administration

  45. When don’t you need a filesystem? Swap space mkswap –v /dev/sdb1 Server applications Oracle VMWare Server CIT 470: Advanced Network and System Administration

  46. Swap Can use swapfile instead of swap partition dd if=/dev/zero of=/swapfile bs=1024k count=512 mkswap /swapfile Enable swap swapon /swapfile swapon /dev/sda2 Disable swap swapoff /swapfile swapoff /dev/sda2 Check swap resource usage cat /proc/swaps CIT 470: Advanced Network and System Administration

  47. Mounting To use a filesystem mount /dev/sda1 /mnt df /mnt Automatic mounting Add an entry in /etc/fstab Unmount umount /dev/sda1 Cannot unmount a volume in use. CIT 470: Advanced Network and System Administration

  48. fstab # /etc/fstab: static file system information. # # <file system> <mount point> <type> <options> <dump> <pass> proc /proc proc defaults 0 0 /dev/hdc1 / ext3 defaults 0 1 /dev/hdc5 /win vfat user,rw 0 0 /dev/hdc7 none swap sw 0 0 /dev/hdc8 /var ext3 defaults 0 2 /dev/hdc9 /home ext3 defaults 0 2 /dev/hda /media/cdrom0 iso9660 ro,user 0 0 /dev/fd0 /media/floppy0 auto rw,user 0 0 CIT 470: Advanced Network and System Administration

  49. UUIDs Universally Unique IDentifiers • 128-bit numbers written as 32 hex digits. • 3.4 × 1038 possible UUIDs Used to identify devices on Linux • To find UUID for a specific device: vol_id –u /dev/sda1 • All devices: ls –l /dev/disk/by-uuid # /etc/fstab: static file system information. # # <file system> <mount point> <type> <options> <dump> <pass> UUID=fbdfebe2-fbde-42c9-963d-12428b642f1d / ext3 defaults 0 1 UUID=a1858e04-78b9-460b-a6cb-3f1dfe3fa16e /home ext3 defaults 0 2 UUID=c4f14e27-96cd-420c-9860-4bd5298e3f76 none swap sw 0 0 CIT 470: Advanced Network and System Administration

  50. fsck: check + repair fs Filesystem corruption sources Power failure System crash Types of corruption Unreferenced inodes. Bad superblocks. Unused data blocks not recorded in block maps. Data blocks listed as free that are used in files. fsck can fix these and more Asks user to make more complex decisions. Stores unfixable files in lost+found. CIT 470: Advanced Network and System Administration

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