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 COMP Feb 2013

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

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

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

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 W W (eg)Hitachi 7k4000Samsung SSD 840

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 COMP OS Device Drivers File System Feb 2013

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” COMP Feb 2013

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

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 COMP Feb 2013

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

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 COMP Feb 2013

Storage Area Network Controller COMP “Blade Servers” SAN Controller Feb 2013

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 COMP Feb 2013

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

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 COMP Feb 2013

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