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 LBPWS and
58 LBPWS10 bits. If LBPWS is set then WRITE SAME (16) supports the UNMAP
59 bit. If LBPWS10 is set then WRITE SAME (10) supports the UNMAP bit. If
60 either LBPWS or LBPWS10 is set and the WRITE SAME (32) is supported
61 then WRITE SAME (32) supports the UNMAP bit.
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. The
70 options are arranged in alphabetical order based on the long option
71 name.
72 -R, --10
74 send a SCSI WRITE SAME (10) command to DEVICE. The ability to
75 set the --unmap (and --anchor) options to this command was added
76 in SBC-3 revision 26.
77 -S, --16
79 send a SCSI WRITE SAME (16) command to DEVICE.
80 -T, --32
82 send a SCSI WRITE SAME (32) command to DEVICE.
83 -a, --anchor
85 sets the ANCHOR bit in the cdb. Introduced in SBC-3 revision 22.
86 That draft requires the --unmap option to also be specified.
87 -g, --grpnum=GN
89 sets the 'Group number' field to GN. Defaults to a value of
90 zero. GN should be a value between 0 and 63.
91 -h, --help
93 output the usage message then exit.
94 -i, --in=IF
96 read data (binary) from file named IF and use it as the data-out
97 buffer for the SCSI WRITE SAME command. The length of the
98 data-out buffer is --xferlen=LEN or, if that is not given, the
99 length of the IF file. If IF is "-" then stdin is read. If this
100 option is not given then 0x00 bytes are used as fill with the
101 length of the data-out buffer obtained from --xferlen=LEN or by
102 calling READ CAPACITY(16 or 10). If the response to READ CAPAC‐
103 ITY(16) has the PROT_EN bit set then data- out buffer size is
104 modified accordingly with the last 8 bytes set to 0xff.
105 -l, --lba=LBA
107 where LBA is the logical block address to start the WRITE SAME
108 command. Defaults to lba 0 which is a dangerous block to over‐
109 write on a disk that is in use. Assumed to be in decimal unless
110 prefixed with '0x' or has a trailing 'h'.
111 -L, --lbdata
113 sets the LBDATA bit in the WRITE SAME cdb. This bit was made
114 obsolete in sbc3r32 in September 2012.
115 -N, --ndob
117 sets the NDOB bit in the WRITE SAME (16 and 32 byte) commands.
118 NDOB stands for No Data-Out Buffer. Default is to clear this
119 bit. When this option is given then --in=IF is not allowed and
120 --xferlen=LEN can only be given if LEN is 0 .
121 -n, --num=NUM
123 where NUM is the number of blocks, starting at LBA, to write the
124 data-out buffer to. The default value for NUM is 1. The value
125 corresponds to the 'Number of logical blocks' field in the WRITE
126 SAME cdb.
127 Note that a value of 0 in NUM may be interpreted as write the
128 data-out buffer on every block starting at LBA to the end of the
129 DEVICE. If the WSNZ bit (introduced in sbc3r26, January 2011)
130 in the Block Limits VPD page is set then the value of 0 is dis‐
131 allowed, yielding an Invalid request sense key.
132 -P, --pbdata
134 sets the PBDATA bit in the WRITE SAME cdb. This bit was made
135 obsolete in sbc3r32 in September 2012.
136 -t, --timeout=TO
138 where TO is the command timeout value in seconds. The default
139 value is 60 seconds. If NUM is large (or zero) a WRITE SAME com‐
140 mand may require considerably more time than 60 seconds to com‐
141 plete.
142 -U, --unmap
144 sets the UNMAP bit in the WRITE SAME(10, 16 and 32) cdb. See
145 UNMAP section below.
146 -v, --verbose
148 increase the degree of verbosity (debug messages).
149 -V, --version
151 output version string then exit.
152 -w, --wrprotect=WPR
154 sets the "Write protect" field in the WRITE SAME cdb to WPR. The
155 default value is zero. WPR should be a value between 0 and 7.
156 When WPR is 1 or greater, and the disk's protection type is 1 or
157 greater, then 8 extra bytes of protection information are
158 expected or generated (to place in the command's data-out buf‐
159 fer).
160 -x, --xferlen=LEN
162 where LEN is the data-out buffer length. Defaults to the length
163 of the IF file or, if that is not given, then the READ CAPAC‐
164 ITY(16 or 10) command is used to find the 'Logical block length
165 in bytes'. That figure may be increased by 8 bytes if the
166 DEVICE's protection type is 1 or greater and the WRPROTECT field
167 (see --wrprotect=WPR) is 1 or greater. If both this option and
168 the IF option are given and LEN exceeds the length of the IF
169 file then LEN is the data-out buffer length with zeros used as
170 pad bytes.
171
173 Logical block provisioning is a new term introduced in SBC-3 revision
174 25 for the ability to mark blocks as unused. For large storage arrays,
175 it is a way to provision less physical storage than the READ CAPACITY
176 command reports is available, potentially allocating more physical
177 storage when WRITE commands require it. For flash memory (e.g. SSD
178 drives) it is a way of potentially saving power (and perhaps access
179 time) when it is known large sections (or almost all) of the flash mem‐
180 ory is not in use. SSDs need wear levelling algorithms to have accept‐
181 able endurance and typically over provision to simplify those algo‐
182 rithms; hence they typically contain more physical flash storage than
183 their logical size would dictate.
184 Support for logical block provisioning is indicated by the LBPME bit
186 being set in the READ CAPACITY(16) command response (see the sg_readcap
187 utility). That implies at least one of the UNMAP or WRITE SAME(16)
188 commands is implemented. If the UNMAP command is implemented then the
189 "Maximum unmap LBA count" and "Maximum unmap block descriptor count"
190 fields in the Block Limits VPD page should both be greater than zero.
191 The READ CAPACITY(16) command response also contains a LBPRZ bit which
192 if set means that if unmapped blocks are read then zeros will be
193 returned for the data (and if protection information is active, 0xff
194 bytes are returned for that). In SBC-3 revision 27 the same LBPRZ bit
195 was added to the Logical Block Provisioning VPD page.
196 In SBC-3 revision 25 the LBPU and ANC_SUP bits where added to the Logi‐
198 cal Block Provisioning VPD page. When LBPU is set it indicates that the
199 device supports the UNMAP command (see the sg_unmap utility). When the
200 ANC_SUP bit is set it indicates the device supports anchored LBAs.
201 When the UNMAP bit is set in the cdb then the data-out buffer is also
203 sent. Additionally the data section of that data-out buffer should be
204 full of 0x0 bytes while the data protection block, 8 bytes at the end
205 if present, should be set to 0xff bytes. If these conditions are not
206 met and the LBPRZ bit is set then the UNMAP bit is ignored and the
207 data-out buffer is written to the DEVICE as if the UNMAP bit was zero.
208 In the absence of the --in=IF option, this utility will attempt build a
209 data-out buffer that meets the requirements for the UNMAP bit in the
210 cdb to be acted on by the DEVICE.
211 Logical blocks may also be unmapped by the SCSI UNMAP and FORMAT UNIT
213 commands (see the sg_unmap and sg_format utilities).
214 The unmap capability in SCSI is closely related to the ATA DATA SET
216 MANAGEMENT command with the "Trim" bit set. That ATA trim capability
217 does not interact well with SATA command queueing known as NCQ. T13
218 have introduced a new command called the SFQ DATA SET MANAGEMENT com‐
219 mand also with a the "Trim" bit to address that problem. The SCSI WRITE
220 SAME with the UNMAP bit set and the UNMAP commands do not have any
221 problems with SCSI queueing.
222
224 Various numeric arguments (e.g. LBA) may include multiplicative suf‐
225 fixes or be given in hexadecimal. See the "NUMERIC ARGUMENTS" section
226 in the sg3_utils(8) man page.
227 In Linux, prior to lk 3.17, the sg driver did not support cdb sizes
229 greater than 16 bytes. Hence a device node like /dev/sg1 which is asso‐
230 ciated with the sg driver would fail with this utility if the --32
231 option was given (or implied by other options). The bsg driver with
232 device nodes like /dev/bsg/6:0:0:1 does support cdb sizes greater than
233 16 bytes since its introduction in lk 2.6.28 .
234
236 The exit status of sg_write_same is 0 when it is successful. Otherwise
237 see the sg3_utils(8) man page.
238
240 One simple usage is to write blocks of zero from (and including) a
241 given LBA:
242 sg_write_same --lba=0x1234 --num=63 /dev/sdc
244 Since --xferlen=LEN has not been given, then this utility will call the
246 READ CAPACITY command on /dev/sdc to determine the number of bytes in a
247 logical block. Let us assume that is 512 bytes. Since --in=IF is not
248 given a block of zeros is assumed. So 63 blocks of zeros (each block
249 containing 512 bytes) will be written from (and including) LBA 0x1234 .
250 Note that only one block of zeros is passed to the SCSI WRITE SAME com‐
251 mand in the data-out buffer (as required by SBC-3).
252 A similar example follows but in this case the blocks are "unmapped"
254 ("trimmed" in ATA speak) rather than zeroed:
255 sg_write_same --unmap -L 0x1234 -n 63 /dev/sdc
257 Note that if the LBPRZ bit in the READ CAPACITY(16) response is set
259 (i.e. LPPRZ is an acronym for logical block provisioning read zeros)
260 then these two examples do the same thing, at least seen from the point
261 of view of subsequent reads.
262 This utility can also be used to write protection information (PI) on
264 disks formatted with a protection type greater than zero. PI is 8 bytes
265 of extra data appended to the user data of a logical block: the first
266 two bytes are a CRC (the "guard"), the next two bytes are the "applica‐
267 tion tag" and the last four bytes are the "reference tag". With protec‐
268 tion types 1 and 2 if the application tag is 0xffff then the guard
269 should not be checked (against the user data).
270 In this example we assume the logical block size (of the user data) is
272 512 bytes and the disk has been formatted with protection type 1. Since
273 we are going to modify LBA 2468 then we take a copy of it first:
274 dd if=/dev/sdb skip=2468 bs=512 of=2468.bin count=1
276 The following command line sets the user data to zeros and the PI to 8
278 0xFF bytes on LBA 2468:
279 sg_write_same --lba=2468 /dev/sdb
281 Reading back that block should be successful because the application
283 tag is 0xffff which suppresses the guard (CRC) check (which would oth‐
284 erwise be wrong):
285 dd if=/dev/sdb skip=2468 bs=512 of=/dev/null count=1
287 Now an attempt is made to create a binary file with zeros in the user
289 data, 0x0000 in the application tag and 0xff bytes in the other two PI
290 fields. It is awkward to create 0xff bytes in a file (in Unix) as the
291 "tr" command below shows:
292 dd if=/dev/zero bs=1 count=512 of=ud.bin
294 tr "\000" "\377" < /dev/zero | dd bs=1 of=ff_s.bin count=8
295 cat ud.bin ff_s.bin > lb.bin
296 dd if=/dev/zero bs=1 count=2 seek=514 conv=notrunc of=lb.bin
297 The resulting file can be viewed with 'hexdump -C lb.bin' and should
299 contain 520 bytes. Now that file can be written to LBA 2468 as follows:
300 sg_write_same --lba=2468 wrprotect=3 --in=lb.bin /dev/sdb
302 Note the --wrprotect=3 rather than being set to 1, since we want the
304 WRITE SAME command to succeed even though the PI data now indicates the
305 user data is corrupted. When an attempt is made to read the LBA, an
306 error should occur:
307 dd if=/dev/sdb skip=2468 bs=512 of=/dev/null count=1
309 dd errors are not very expressive, if dmesg is checked there should be
311 a line something like this: "[sdb] Add. Sense: Logical block guard
312 check failed". The block can be corrected by doing a "sg_write_same
313 --lba=1234 /dev/sdb" again or restoring the original contents of that
314 LBA:
315 dd if=2468.bin bs=512 seek=2468 of=/dev/sdb conv=notrunc count=1
317 Hopefully the dd command would never try to truncate the output file
319 when it is a block device.
320
322 Written by Douglas Gilbert.
323
325 Report bugs to <dgilbert at interlog dot com>.
326
328 Copyright © 2009-2017 Douglas Gilbert
329 This software is distributed under a FreeBSD license. There is NO war‐
330 ranty; not even for MERCHANTABILITY or FITNESS FOR A PARTICULAR PUR‐
331 POSE.
332
334 sg_format,sg_get_lba_status,sg_readcap,sg_vpd,sg_unmap(sg3_utils)
335sg3_utils-1.43 November 2017 SG_WRITE_SAME(8)