Ch 13 Block I/O Layer

1. 제50강 : Block IO Layer Ch 13 Block I/O Layer

2. Vectored I/O(scatter-read, gather-write) P1 text different permission (R W X ..)| different page text P2 a.out data data lib P3 lib

3. Reminder – Lion’s code • bread(buf) 4754 • call bdevsw[].d_strategy(rbp) 4763 • type bdevsw[] 4617 • init bdevsw[] 4658  &rkstrategy() • rkstrategy() 5389 Removable Disk • 5410 insert buf at front (actf) • 5412 insert buf at last (actl) • 5415 if disk is idle, … • ------------------------------------ • invoke bread() N times if vectored I/O? • Disk head scheduling

4. text text data data lib lib

5. Offset lbn bn readi() 6221 6221 readi(aip) 6222 struct inode *aip; 6224 { int *bp; 6225 int lbn, bn, on; 6226 register dn, n; 6227 register struct inode *ip; 6229 ip = aip; 6230 if(u.u_count == 0) return; 6232 ip->i_flag =| IACC; /* inode = “accessed” */ 6233 if((ip->i_mode&IFMT) == IFCHR) { /* character file? */ • (*cdevsw[ip->i_addr[0].d_major].d_read)(ip->i_addr[0]); • return; • } 6238 do { lbn = bn = lshift(u.u_offset, -9); 6240 on = u.u_offset[1] & 0777; 6241 n = min(512-on, u.u_count); 6242 if((ip->i_mode&IFMT) != IFBLK) { 6243 dn = dpcmp(ip->i_size0&0377, ip->i_size1, 6244 u.u_offset[0], u.u_offset[1]); 6245 if(dn <= 0) return; 6247 n = min(n, dn); 6248 if ((bn = bmap(ip, lbn)) == 0) return; 6250 dn = ip->i_dev; 6251 } else { 6252 dn = ip->i_addr[0]; 6253 rablock = bn+1; 6254 } 6255 if (ip->i_lastr+1 == lbn) 6256 bp = breada(dn, bn, rablock); 6257 else 6258 bp = bread(dn, bn); 6259 ip->i_lastr = lbn; 6260 iomove(bp, on, n, B_READ); 6261 brelse(bp); 6262 } while(u.u_error==0 && u.u_count!=0); 6263 }

6. buf [NBUF] 4535

7. rktab: (5386) d_actf d_actl rkstrategy() 5389 rkstrategy(abp) 5390 struct buf *abp; 5391 { 5392 register struct buf *bp; 5393 register *qc, *ql; 5394 int d; 5395 5396 bp = abp; 5397 if(bp->b_flags&B_PHYS) 5398 mapalloc(bp); 5399 d = bp->b_dev.d_minor-7; 5400 if(d <= 0) 5401 d = 1; 5402 if (bp->b_blkno >= NRKBLK*d) { 5403 bp->b_flags =| B_ERROR; 5404 iodone(bp); 5405 return; 5406 } 5407 bp->av_forw = 0; 5408 spl5(); 5409 if (rktab.d_actf==0) 5410 rktab.d_actf = bp; / 5411 else 5412 rktab.d_actl->av_forw = bp; 5413 rktab.d_actl = bp; 5414 if (rktab.d_active==0) 5415 rkstart(); 5416 spl0(); 5417 } text text data data lib lib

8. Lion’s code buffer_header 4520: struct buf 4521: { 4522: int b_flags; 4523: struct buf *b_forw; 4524: struct buf *b_back; 4525: struct buf *av_forw; 4526: struct buf *av_back; 4527: int b_dev; 4528: int b_wcount; 4529: char *b_addr; (low) 4530: char *b_xmem; (high) 4531: char *b_blkno; 4532: char b_error; 4533: char *b_resid; 4534: 4535: } buf[NBUF]; text text data data lib lib b_dev Memory sector low address b_blkno high address buffer

9. Problems with Lion’s code Buffer Head text • [Time] • For vectored I/O, call N times. • [Space] • N buffer headers – one for each segment • Much information - replicated • Size of buffer head(metadata) was large • whereas size of buffer wassmall text data data lib lib

10. memory page bio text Page 0 Offset Length text Page 1 Offset Length data data lib Page 2 Offset length lib The bio structure • represents N block I/O operations • which consists of a list of N segments • each segment may be in separate locations in memory • Separate Roles: • “struct bio” (new) • represents N I/O operations • “buffer head” • provide mapping • memory buffer vs disk sector

11. memory page bio Page 0 Offset Length text text page structures involved in block I/O operation Page 1 Offset Length data data lib Page 2 Offset length lib The bio structure bi_io_vec struct bio_vec { struct page *bv_page; unsigned int bv_len; unsigned int bv_offset; }; bio_vec bi_idx bio_vec struct bio list of bio_vec structures bio_vec adjacent in disk separate in memory

12. struct bio { sector_t bi_sector; struct bio *bi_next; /* request queue link */ struct block_device *bi_bdev; unsigned long bi_flags; /* status, command, etc */ unsigned short bi_vcnt; /* how many bio_vec's */ unsigned short bi_idx; /* current index into bvl_vec */ unsigned short bi_hw_segments; unsigned int bi_size; /* residual I/O count */ unsigned int bi_max_vecs; /* max bvl_vecs we can hold */ struct bio_vec *bi_io_vec; /* the actual vec list */ bio_end_io_t *bi_end_io; atomic_t bi_cnt; /* pin count */ void *bi_private; bio_destructor_t *bi_destructor; /* destructor */ } bio_vec struct bio_vec { struct page *bv_page; unsigned int bv_len; unsigned int bv_offset; }; struct bio bio_vec list of bio_vec structures bio_vec

13. Individual I/O Request text text ppt a.out data data lib lib data file data data

14. Individual I/O Request I/O request i bio_vec (text) bio_vec (data) bio_vec (…) bio 1 struct request { …… struct bio *cbio; /* next bio to submit */ struct bio *bio; /* next unfinished bio to complete */ struct bio *biotail; void *elevator_private; int rq_status; struct gendisk *rq_disk; int errors; unsigned long start_time; ……….. unsigned short nr_phys_segments; int tag; char *buffer; int ref_count; request_queue_t *q; …… } bio 2 bio_vec (text) bio_vec (data) bio_vec (…) bio 3

15. Request Queue per Device Disk request queue • pending block I/O requests on block device. • list of struct request • each request -- more than one bio structure • control & status information • struct request_queue in <linux/blkdev.h> I/O request 1 bio_vec bio_vec bio_vec bio I/O request 2 bio bio_vec bio_vec bio_vec I/O request 3

16. I/O Scheduler • The Linus Elevator • If a request to an adjacent on-disk sector is in the queue, two adjacent requests are merged into a single request • insert new request into the queue at a spot near other requests operating on physically near sectors • If a request in the queue is sufficiently old, the new request is inserted at the tail (prevent starvation)

17. 2 types of device • Block Device • Random access • fixed size chunk of data • navigate back & forth any location on media • eg disk for word file • Character Device • Sequential access of stream of data (no back & forth allowed) • eg keyboard, swap disk