![BITS Pilani, Pilani Campus
Local File Store – Layout [1]
• Global Policies:
– Use summary information to make decisions
regarding placement of i-nodes and disk blocks.
• Routines responsible for deciding placement of new
directories and files.
– Layout policies rely on locality to cluster information
for improved performance.
• E.g. Cylinder group clustering
• Local Allocation Routines.
– Use locally optimal schemes for data block layouts.
5](https://image.slidesharecdn.com/m2-212-unixfsrl2-180108063342/85/M2-212-unix-fs-rl_2-1-2-5-320.jpg)
![BITS Pilani, Pilani Campus
Local File Store – Layout [2]
• Local allocators use a multi-level allocation strategy:
1. Use the next available block that is rotationally
closest to the requested block on the same cylinder
2. If no blocks are available in the same cylinder use
a block within the same cylinder group
3. If the cylinder group is full, choose another group
by quadratic hashing.
4. Search exhaustively.
6](https://image.slidesharecdn.com/m2-212-unixfsrl2-180108063342/85/M2-212-unix-fs-rl_2-1-2-6-320.jpg)
M2 212-unix fs-rl_2.1.2
- 1. BITS Pilani Pilani Campus Data Storage Technologies & Networks Dr. Virendra Singh Shekhawat Department of Computer Science and Information Systems
- 2. BITS Pilani, Pilani Campus Topics • File Store Organization • OS Support for I/O – Device Drivers – Interrupt handling – Device Interface – Buffering 2
- 3. BITS Pilani, Pilani Campus Organization of File store • Berkeley Fast File System (FFS) model: – A file system is described by its superblock • Located at the beginning of the file system • Possibly replicated for redundancy – A disk partition is divided into one or more cylinder groups i.e. a set of consecutive cylinders: • Each group maintains book-keeping info. including – a redundant copy of superblock – Space for i-nodes – A bitmap of available blocks and – Summary usage info. • Cylinder groups are used to create clusters of allocated blocks to improve locality. 3
- 4. BITS Pilani, Pilani Campus Local File Store- Storage Utilization • Data layout – Performance requirement – Large blocks of data should be transferable in a single disk operation • So, logical block size must be large. – But, typical profiles primarily use small files. • Internal Fragmentation: – Increases from 1.1% for 512 bytes logical block size, 2.5% for 1KB, to 5.4% for 2KB, to an unacceptable 61% for 16KB • I-nodes also add to the space overhead: – But overhead due to i-nodes is about 6% for logical block sizes of 512B, 1KB and 2KB, reduces to about 3% for 4KB, and to 0.8% for 16KB. • One option to balance internal fragmentation against improved I/O performance is – to maintain large logical blocks made of smaller fragments 4
- 5. BITS Pilani, Pilani Campus Local File Store – Layout [1] • Global Policies: – Use summary information to make decisions regarding placement of i-nodes and disk blocks. • Routines responsible for deciding placement of new directories and files. – Layout policies rely on locality to cluster information for improved performance. • E.g. Cylinder group clustering • Local Allocation Routines. – Use locally optimal schemes for data block layouts. 5
- 6. BITS Pilani, Pilani Campus Local File Store – Layout [2] • Local allocators use a multi-level allocation strategy: 1. Use the next available block that is rotationally closest to the requested block on the same cylinder 2. If no blocks are available in the same cylinder use a block within the same cylinder group 3. If the cylinder group is full, choose another group by quadratic hashing. 4. Search exhaustively. 6
- 7. BITS Pilani, Pilani Campus OS Support for I/O • System calls form the interface between applications and OS (kernel) – File System and I/O system are responsible for • Implementing system calls related to file management and handling input/output. • Device drivers form the interface between the OS (kernel) and the hardware 7
- 8. BITS Pilani, Pilani Campus I/O in UNIX - Example • Application level operation – E.g. printf call • OS (kernel) level – System call bwrite • Device Driver level – Strategy entry point – code for write operation • Device level – E.g. SCSI protocol command for write 8
- 9. BITS Pilani, Pilani Campus I/O in UNIX - Devices • I/O system is used for – Network communication and virtual memory (Swap space) • Two types of devices – Character devices • Terminals, line printers, main memory – Block devices • Disks and Tapes • Buffering done by kernel 9
- 10. BITS Pilani, Pilani Campus I/O in UNIX – Device Drivers • Device Driver Sections – Auto-configuration and initialization Routines • Probe and initialize the device – Routines for servicing I/O requests • Invoked because of system calls or VM requests • Referred to as the “top-half” of the driver – Interrupt service routines • Invoked by interrupt from a device – Can’t depend on per-process state – Can’t block • Referred to as the “bottom-half” of the driver 10
- 11. BITS Pilani, Pilani Campus I/O Queuing and Interrupt Handling • Each device driver manages one or more queues for I/O requests – Shared among asynchronous routines – must be synchronized. – Multi-process synchronization also required. • Interrupts are generated by devices – Signal status change (or completion of operation) – DD-ISR invoked through a glue routine that is responsible for saving volatile registers. – ISR removes request from queue, notifies requestor that the command has been executed. 11
- 12. BITS Pilani, Pilani Campus I/O in UNIX – Block Devices and I/O • Disk sector sized read/write – Conversion of random access to disk sector read/write is known as block I/O – Kernel buffering reduces latency for multiple reads and writes. 12
- 13. BITS Pilani, Pilani Campus Device Driver to Disk Interface • Disk Interface: – Disk access requires an address: • device id, LBA OR • device id, cylinder #, head#, sector #. – Device drivers need to be aware of disk details for address translation: • i.e. converting a logical address (say file system level address such as i-node number) to a physical address (i.e. CHS) on the disk; • they need to be aware of complete disk geometry if CHS addressing is used. – Early device drivers had hard-coded disk geometries. • This results in reduced modularity – – disks cannot be moved (data mapping is with the device driver); – device driver upgrades would require shutdowns and data copies. 13
- 14. BITS Pilani, Pilani Campus Disk Labels • Disk Geometry and Data mapping is stored on the disk itself. – Known as disk label. – Must be stored in a fixed location – usually 0th block i.e. 0th sector on head 0, cylinder 0. – Usually this is where bootstrap code is stored • In which case disk information is part of the bootstrap code, because booting may require disk access. 14
- 15. BITS Pilani, Pilani Campus Buffering • Buffer Cache – Memory buffer for data being transferred to and from disks – Cache for recently used blocks • 85% hit rate is typical • Typical buffer size = 64KB virtual memory • Buffer pool – hundreds of buffers • Consistency issue – Each disk block mapped to at most one buffer – Buffers have dirty bits associated – When a new buffer is allocated and if its disk blocks overlap with that of an existing buffer, then the old buffer must be purged. 15
- 16. BITS Pilani, Pilani Campus Buffer Pool Management • Buffer pool is maintained as a (separately chained) hash table indexed by a buffer id • The buffers in the pool are also in one of four lists: – Locked list: • buffers that are currently used for I/O and therefore locked and cannot be released until operation is complete – LRU list: • A queue of buffers – a recently used item is added at the rear of the queue and when a buffer is needed one at the front of the queue is replaced. • Buffers staying in this queue long enough are migrated to an Aged list – Aged List: • Maintained as a list and any element may be used for replacement. – Empty List • When a new buffer is needed check in the following order: – Empty List, Aged List, LRU list 16
- 17. BITS Pilani, Pilani Campus Thank You! 17