1. Storage Systems: Advanced Topics Learning Objectives: To understand major characteristics of SSD To understand Logical Volume Management – its motivations and its facilities To understand how Storage Area Networks extend these features to multiple systems To relate LVM and SAN to a most modern file system implementation COMP25212 1 Feb 2013

2. Solid State Disks Today’s technology: Flash Memory –Floating gate Field Effect Transistor COMP25212 Store {0, 1} on “Floating” gate - no electrical connection Feb 2013

3. NAND Flash Operation No connection to “Floating” gate –Quantum tunneling –Block erase, bit writing –Memory wear (10K writes?) –Multi Level Cell e.g. {0, ¼, ½, ¾} 2 bits/cell –Read disturb COMP25212Feb 2013

4. Flash Controller Implement: –Error Correcting Codes –(Bad) Block Remapping –Write levelling –Read tracking and rewriting But note: –10 year data retention(?) –Wear-out with write cycles –Performance falls as file system fills –Performance falls with device wearout Feb 2013COMP25212

5. Hard Disks and SSD Feb 2013COMP25212 (2013 figures)Hard DiskSSD Capacity/price4Tb/£140250Gb/£125 Streaming reads205 Mb/s530 Mb/s Streaming writes205 Mb/s240 Mb/s Random 4Kb read15.5 mS11 uS Random 4Kb write6.4 mS23 uS Power6 - 8 W0.3 - 4.2 W (eg)Hitachi 7k4000Samsung SSD 840

6. Storage Virtualization File System Code –assumes 1 file system –to 1 disk drive (partition) Stripe/Mirror/RAID changes this Storage Virtualization: break filesystem/drive relationship COMP25212 6 OS Device Drivers File System Feb 2013

7. Logical Volume Management Virtual mapping between file system code and physical device Analogous (but not identical!) to virtual memory addressing “Volume Group” of all drives in a pool Storage space in “Volume Group” divided into “Physical Extents” – usually all same size “Logical Volume” is set of “Physical Extents” COMP25212 7 Feb 2013

8. Logical Volume Management Mirror/Stripe/RAID provided within LVM layer Resize filesystem “Snapshot” a live filesystem COMP25212 8 OS Device Drivers File System LVM layer Volume Group Feb 2013

9. e.g. Linux Server / - mostly read – want fast seeks swap – read/write – want high bandwidth /opt – infrequent access /var – huge, infrequent access Mirror / Stripe swap spare space to /opt and /var COMP25212 9 Feb 2013

10. LVM Example: Now we can share resources more flexibly and more efficiently – but only on one server COMP25212 10 Mirror / Stripe swap /var /opt Feb 2013

11. Storage Area Networking Implement LVM features in separate storage controller Connect multiple servers to storage controller via SCSI, or FibreChannel, or Infiniband, or … (n.b. SAN over Ethernet is conventionally called Networked Attached Storage (NAS) Now can share disk resources across multiple servers Rapid migration of disk images COMP25212 11 Feb 2013

12. Storage Area Network Controller COMP25212 12 8 “Blade Servers” SAN Controller Feb 2013

13. SAN Key Features Functionality –Key element of “system virtualization” –Migrating virtual machines –“De-duping” – share common subsets of file systems (think Virtual Machine images!) Management: –Manage storage separately from server physical resources –Maximize flexibility of storage provisioning COMP25212 13 Feb 2013

14. ZFS – Volume Aware File System Marketing claims: Lost a file? Run out of space? Difficult disk upgrade? Want to grow/shrink? Data Corruption? COMP25212 14 Pools of storage Feb 2013

15. ZFS techniques Lost a file? –Copy-on-write – simple rollback/recovery Run out of space/Difficult disk upgrade? –Add new storage to live systems –Self-checking, self-healing Want to grow/shrink? Data Corruption? –end-to-end sumchecking COMP25212 15 Feb 2013

16. ZFS Implementation ZFS Combines File Service and Logical Volume Management Is this a good idea? Discuss COMP25212 16 Feb 2013