1SG_WRITE_SAME(8) SG3_UTILS SG_WRITE_SAME(8)
2
6 sg_write_same - send SCSI WRITE SAME command
7
9 sg_write_same [--10] [--16] [--32] [--anchor] [--grpnum=GN] [--help]
10 [--in=IF] [--lba=LBA] [--lbdata] [--num=NUM] [--ndob] [--pbdata]
11 [--timeout=TO] [--unmap] [--verbose] [--version] [--wrprotect=WPR]
12 [--xferlen=LEN] DEVICE
13
15 Send the SCSI WRITE SAME (10, 16 or 32 byte) command to DEVICE. This
16 command writes the given block NUM times to consecutive blocks on the
17 DEVICE starting at logical block address LBA.
18 The length of the block to be written multiple times is obtained from
20 either the LEN argument, or the length of the given input file IF, or
21 by calling READ CAPACITY(16) on DEVICE. The contents of the block to be
22 written are obtained from the input file IF or zeros are used. If READ
23 CAPACITY(16) is called (which implies IF was not given) and the PROT_EN
24 bit is set then an extra 8 bytes (i.e. more than the logical block
25 size) of 0xff are sent. If READ CAPACITY(16) fails then READ CAPAC‐
26 ITY(10) is used to determine the block size.
27 If neither --10, --16 nor --32 is given then WRITE SAME(10) is sent
29 unless one of the following conditions is met. If LBA (plus NUM)
30 exceeds 32 bits, NUM exceeds 65535, or the --unmap option is given then
31 WRITE SAME(16) is sent. The --10, --16 and --32 options are mutually
32 exclusive.
33 SBC-3 revision 35d introduced a "no data-out buffer" (NDOB) bit which,
35 if set, bypasses the requirement to send a single block of data to the
36 DEVICE together with the command. Only WRITE SAME (16 and 32 byte) sup‐
37 port the NDOB bit. If given, a user block of zeros is assumed; if
38 required, protection information of 0xffs is assumed.
39 In SBC-3 revision 26 the UNMAP and ANCHOR bits were added to the WRITE
41 SAME (10) command. Since the UNMAP bit has been in WRITE SAME (16) and
42 WRITE SAME (32) since SBC-3 revision 18, the lower of the two (i.e.
43 WRITE SAME (16)) is the default when the --unmap option is given. To
44 send WRITE SAME (10) use the --10 option.
45 Take care: The WRITE SAME(10, 16 and 32) commands may interpret a NUM
47 of zero as write to the end of DEVICE. This utility defaults NUM to 1 .
48 The WRITE SAME commands have no IMMED bit so if NUM is large (or zero)
49 then an invocation of this utility could take a long time, potentially
50 as long as a FORMAT UNIT command. In such situations the command time‐
51 out value TO may need to be increased from its default value of 60 sec‐
52 onds. In SBC-3 revision 26 the WSNZ (write same no zero) bit was added
53 to the Block Limits VPD page [0xB0]. If set the WRITE SAME commands
54 will not accept a NUM of zero. The same SBC-3 revision added the "Maxi‐
55 mum Write Same Length" field to the Block Limits VPD page.
56 The Logical Block Provisioning VPD page [0xB2] contains the LBWS and
58 LBW10 bits. If LBWS is set then WRITE SAME (16) supports the UNMAP bit.
59 If LBWS10 is set then WRITE SAME (10) supports the UNMAP bit. If either
60 LBWS or LBWS10 is set and the WRITE SAME (32) is supported then WRITE
61 SAME (32) supports the UNMAP bit. This is as of SBC-3 revision 26.
62 As a precaution against an accidental 'sg_write_same /dev/sda' (for
64 example) overwriting LBA 0 on /dev/sda with zeros, at least one of the
65 --in=IF, --lba=LBA or --num=NUM options must be given. Obviously this
66 utility can destroy a lot of user data so check the options carefully.
67
69 Arguments to long options are mandatory for short options as well.
70 -R, --10
72 send a SCSI WRITE SAME (10) command to DEVICE. The ability to
73 set the --unmap (and --anchor) options to this command was added
74 in SBC-3 revision 26.
75 -S, --16
77 send a SCSI WRITE SAME (16) command to DEVICE.
78 -T, --32
80 send a SCSI WRITE SAME (32) command to DEVICE.
81 -a, --anchor
83 sets the ANCHOR bit in the cdb. Introduced in SBC-3 revision 22.
84 That draft requires the --unmap option to also be specified.
85 -g, --grpnum=GN
87 sets the 'Group number' field to GN. Defaults to a value of
88 zero. GN should be a value between 0 and 31.
89 -h, --help
91 output the usage message then exit.
92 -i, --in=IF
94 read data (binary) from file named IF and use it as the data out
95 buffer for the SCSI WRITE SAME command. The length of the data
96 out buffer is --xferlen=LEN or, if that is not given, the length
97 of the IF file. If IF is "-" then stdin is read. If this option
98 is not given then 0x00 bytes are used as fill with the length of
99 the data out buffer obtained from --xferlen=LEN or by calling
100 READ CAPACITY(16 or 10). If the response to READ CAPACITY(16)
101 has the PROT_EN bit set then data out buffer size is modified
102 accordingly with the last 8 bytes set to 0xff.
103 -l, --lba=LBA
105 where LBA is the logical block address to start the WRITE SAME
106 command. Defaults to lba 0 which is a dangerous block to over‐
107 write on a disk that is in use. Assumed to be in decimal unless
108 prefixed with '0x' or has a trailing 'h'.
109 -L, --lbdata
111 sets the LBDATA bit in the WRITE SAME cdb. This bit was made
112 obsolete in sbc3r32 in September 2012.
113 -N, --ndob
115 sets the NDOB bit in the WRITE SAME (16 and 32 byte) commands.
116 Default is to clear this bit. When this option is given then
117 --in=IF is not allowed and --xferlen=LEN can only be given if
118 LEN is 0 .
119 -n, --num=NUM
121 where NUM is the number of blocks, starting at LBA, to write the
122 data out buffer to. The default value for NUM is 1. The value
123 corresponds to the 'Number of logical blocks' field in the WRITE
124 SAME cdb.
125 Note that a value of 0 in NUM may be interpreted as write the
126 data out buffer on every block starting at LBA to the end of the
127 DEVICE. If the WSNZ bit (introduced in sbc3r26, January 2011)
128 in the Block Limits VPD page is set then the value of 0 is dis‐
129 allowed, yielding an Invalid request sense key.
130 -P, --pbdata
132 sets the PBDATA bit in the WRITE SAME cdb. This bit was made
133 obsolete in sbc3r32 in September 2012.
134 -t, --timeout=TO
136 where TO is the command timeout value in seconds. The default
137 value is 60 seconds. If NUM is large (or zero) a WRITE SAME com‐
138 mand may require considerably more time than 60 seconds to com‐
139 plete.
140 -U, --unmap
142 sets the UNMAP bit in the WRITE SAME(10, 16 and 32) cdb. See
143 UNMAP section below.
144 -v, --verbose
146 increase the degree of verbosity (debug messages).
147 -V, --version
149 output version string then exit.
150 -w, --wrprotect=WPR
152 sets the "Write protect" field in the WRITE SAME cdb to WPR. The
153 default value is zero. WPR should be a value between 0 and 7.
154 When WPR is 1 or greater, and the disk's protection type is 1 or
155 greater, then 8 extra bytes of protection information are
156 expected or generated (to place in the command's data out buf‐
157 fer).
158 -x, --xferlen=LEN
160 where LEN is the data out buffer length. Defaults to the length
161 of the IF file or, if that is not given, then the READ CAPAC‐
162 ITY(16 or 10) command is used to find the 'Logical block length
163 in bytes'. That figure may be increased by 8 bytes if the
164 DEVICE's protection type is 1 or greater and the WRPROTECT field
165 (see --wrprotect=WPR) is 1 or greater. If both this option and
166 the IF option are given and LEN exceeds the length of the IF
167 file then LEN is the data out buffer length with zeros used as
168 pad bytes.
169
171 Logical block provisioning is a new term introduced in SBC-3 revision
172 25 for the ability to mark blocks as unused. For large storage arrays,
173 it is a way to provision less physical storage than the READ CAPACITY
174 command reports is available, potentially allocating more physical
175 storage when WRITE commands require it. For flash memory (e.g. SSD
176 drives) it is a way of potentially saving power (and perhaps access
177 time) when it is known large sections (or almost all) of the flash mem‐
178 ory is not in use. SSDs need wear levelling algorithms to have accept‐
179 able endurance and typically over provision to simplify those algo‐
180 rithms; hence they typically contain more physical flash storage than
181 their logical size would dictate.
182 Support for logical block provisioning is indicated by the LBPME bit
184 being set in the READ CAPACITY(16) command response (see the sg_readcap
185 utility). That implies at least one of the UNMAP or WRITE SAME(16)
186 commands is implemented. If the UNMAP command is implemented then the
187 "Maximum unmap LBA count" and "Maximum unmap block descriptor count"
188 fields in the Block Limits VPD page should both be greater than zero.
189 The READ CAPACITY(16) command response also contains a LBPRZ bit which
190 if set means that if unmapped blocks are read then zeros will be
191 returned for the data (and if protection information is active, 0xff
192 bytes are returned for that). In SBC-3 revision 27 the same LBPRZ bit
193 was added to the Logical Block Provisioning VPD page.
194 In SBC-3 revision 25 the LBPU and ANC_SUP bits where added to the Logi‐
196 cal Block Provisioning VPD page. When LBPU is set it indicates that the
197 device supports the UNMAP command (see the sg_unmap utility). When the
198 ANC_SUP bit is set it indicates the device supports anchored LBAs.
199 When the UNMAP bit is set in the cdb then the data out buffer is also
201 sent. Additionally the data section of that data out buffer should be
202 full of 0x0 bytes while the data protection block, 8 bytes at the end
203 if present, should be set to 0xff bytes. If these conditions are not
204 met and the LBPRZ bit is set then the UNMAP bit is ignored and the data
205 out buffer is written to the DEVICE as if the UNMAP bit was zero. In
206 the absence of the --in=IF option, this utility will attempt build a
207 data out buffer that meets the requirements for the UNMAP bit in the
208 cdb to be acted on by the DEVICE.
209 Logical blocks may also be unmapped by the SCSI UNMAP and FORMAT UNIT
211 commands (see the sg_unmap and sg_format utilities).
212 The unmap capability in SCSI is closely related to the ATA DATA SET
214 MANAGEMENT command with the "Trim" bit set. That ATA trim capability
215 does not interact well with SATA command queueing known as NCQ. T13
216 have introduced a new command called the SFQ DATA SET MANAGEMENT com‐
217 mand also with a the "Trim" bit to address that problem. The SCSI WRITE
218 SAME with the UNMAP bit set and the UNMAP commands do not have any
219 problems with SCSI queueing.
220
222 Various numeric arguments (e.g. LBA) may include multiplicative suf‐
223 fixes or be given in hexadecimal. See the "NUMERIC ARGUMENTS" section
224 in the sg3_utils(8) man page.
225 In Linux, prior to lk 3.17, the sg driver did not support cdb sizes
227 greater than 16 bytes. Hence a device node like /dev/sg1 which is asso‐
228 ciated with the sg driver would fail with this utility if the --32
229 option was given (or implied by other options). The bsg driver with
230 device nodes like /dev/bsg/6:0:0:1 does support cdb sizes greater than
231 16 bytes since its introduction in lk 2.6.28 .
232
234 The exit status of sg_write_same is 0 when it is successful. Otherwise
235 see the sg3_utils(8) man page.
236
238 One simple usage is to write blocks of zero from (and including) a
239 given LBA:
240 sg_write_same --lba=0x1234 --num=63 /dev/sdc
242 Since --xferlen=LEN has not been given, then this utility will call the
244 READ CAPACITY command on /dev/sdc to determine the number of bytes in a
245 logical block. Let us assume that is 512 bytes. Since --in=IF is not
246 given a block of zeros is assumed. So 63 blocks of zeros (each block
247 containing 512 bytes) will be written from (and including) LBA 0x1234 .
248 Note that only one block of zeros is passed to the SCSI WRITE SAME com‐
249 mand in the data out buffer (as required by SBC-3).
250 A similar example follows but in this case the blocks are "unmapped"
252 ("trimmed" in ATA speak) rather than zeroed:
253 sg_write_same --unmap -L 0x1234 -n 63 /dev/sdc
255 Note that if the LBPRZ bit in the READ CAPACITY(16) response is set
257 (i.e. LPPRZ is an acronym for logical block provisioning read zeros)
258 then these two examples do the same thing, at least seen from the point
259 of view of subsequent reads.
260 This utility can also be used to write protection information (PI) on
262 disks formatted with a protection type greater than zero. PI is 8 bytes
263 of extra data appended to the user data of a logical block: the first
264 two bytes are a CRC (the "guard"), the next two bytes are the "applica‐
265 tion tag" and the last four bytes are the "reference tag". With protec‐
266 tion types 1 and 2 if the application tag is 0xffff then the guard
267 should not be checked (against the user data).
268 In this example we assume the logical block size (of the user data) is
270 512 bytes and the disk has been formatted with protection type 1. Since
271 we are going to modify LBA 2468 then we take a copy of it first:
272 dd if=/dev/sdb skip=2468 bs=512 of=2468.bin count=1
274 The following command line sets the user data to zeros and the PI to 8
276 0xFF bytes on LBA 2468:
277 sg_write_same --lba=2468 /dev/sdb
279 Reading back that block should be successful because the application
281 tag is 0xffff which suppresses the guard (CRC) check (which would oth‐
282 erwise be wrong):
283 dd if=/dev/sdb skip=2468 bs=512 of=/dev/null count=1
285 Now an attempt is made to create a binary file with zeros in the user
287 data, 0x0000 in the application tag and 0xff bytes in the other two PI
288 fields. It is awkward to create 0xff bytes in a file (in Unix) as the
289 "tr" command below shows:
290 dd if=/dev/zero bs=1 count=512 of=ud.bin
292 tr "\000" "\377" < /dev/zero | dd bs=1 of=ff_s.bin count=8
293 cat ud.bin ff_s.bin > lb.bin
294 dd if=/dev/zero bs=1 count=2 seek=514 conv=notrunc of=lb.bin
295 The resulting file can be viewed with 'hexdump -C lb.bin' and should
297 contain 520 bytes. Now that file can be written to LBA 2468 as follows:
298 sg_write_same --lba=2468 wrprotect=3 --in=lb.bin /dev/sdb
300 Note the --wrprotect=3 rather than being set to 1, since we want the
302 WRITE SAME command to succeed even though the PI data now indicates the
303 user data is corrupted. When an attempt is made to read the LBA, an
304 error should occur:
305 dd if=/dev/sdb skip=2468 bs=512 of=/dev/null count=1
307 dd errors are not very expressive, if dmesg is checked there should be
309 a line something like this: "[sdb] Add. Sense: Logical block guard
310 check failed". The block can be corrected by doing a "sg_write_same
311 --lba=1234 /dev/sdb" again or restoring the original contents of that
312 LBA:
313 dd if=2468.bin bs=512 seek=2468 of=/dev/sdb conv=notrunc count=1
315 Hopefully the dd command would never try to truncate the output file
317 when it is a block device.
318
320 Written by Douglas Gilbert.
321
323 Report bugs to <dgilbert at interlog dot com>.
324
326 Copyright © 2009-2015 Douglas Gilbert
327 This software is distributed under a FreeBSD license. There is NO war‐
328 ranty; not even for MERCHANTABILITY or FITNESS FOR A PARTICULAR PUR‐
329 POSE.
330
332 sg_format,sg_get_lba_status,sg_readcap,sg_vpd,sg_unmap(sg3_utils)
333sg3_utils-1.41 February 2015 SG_WRITE_SAME(8)