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] [--ff] [--grpnum=GN]
10 [--help] [--in=IF] [--lba=LBA] [--lbdata] [--num=NUM] [--ndob]
11 [--pbdata] [--timeout=TO] [--unmap] [--verbose] [--version] [--wrpro‐
12 tect=WPR] [--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 -f, --ff
89 the data-out buffer sent with this command is initialized with
90 0xff bytes when this option is given.
91 -g, --grpnum=GN
93 sets the 'Group number' field to GN. Defaults to a value of
94 zero. GN should be a value between 0 and 63.
95 -h, --help
97 output the usage message then exit.
98 -i, --in=IF
100 read data (binary) from file named IF and use it as the data-out
101 buffer for the SCSI WRITE SAME command. The length of the
102 data-out buffer is --xferlen=LEN or, if that is not given, the
103 length of the IF file. If IF is "-" then stdin is read. If this
104 option and the --ff are not given then 0x00 bytes are used as
105 fill with the length of the data-out buffer obtained from
106 --xferlen=LEN or by calling READ CAPACITY(16 or 10). If the
107 response to READ CAPACITY(16) has the PROT_EN bit set then data-
108 out buffer size is modified accordingly with the last 8 bytes
109 set to 0xff.
110 -l, --lba=LBA
112 where LBA is the logical block address to start the WRITE SAME
113 command. Defaults to lba 0 which is a dangerous block to over‐
114 write on a disk that is in use. Assumed to be in decimal unless
115 prefixed with '0x' or has a trailing 'h'.
116 -L, --lbdata
118 sets the LBDATA bit in the WRITE SAME cdb. This bit was made
119 obsolete in sbc3r32 in September 2012.
120 -N, --ndob
122 sets the NDOB bit in the WRITE SAME (16 and 32 byte) commands.
123 NDOB stands for No Data-Out Buffer. Default is to clear this
124 bit. When this option is given then --in=IF is not allowed and
125 --xferlen=LEN can only be given if LEN is 0 .
126 -n, --num=NUM
128 where NUM is the number of blocks, starting at LBA, to write the
129 data-out buffer to. The default value for NUM is 1. The value
130 corresponds to the 'Number of logical blocks' field in the WRITE
131 SAME cdb.
132 Note that a value of 0 in NUM may be interpreted as write the
133 data-out buffer on every block starting at LBA to the end of the
134 DEVICE. If the WSNZ bit (introduced in sbc3r26, January 2011)
135 in the Block Limits VPD page is set then the value of 0 is dis‐
136 allowed, yielding an Invalid request sense key.
137 -P, --pbdata
139 sets the PBDATA bit in the WRITE SAME cdb. This bit was made
140 obsolete in sbc3r32 in September 2012.
141 -t, --timeout=TO
143 where TO is the command timeout value in seconds. The default
144 value is 60 seconds. If NUM is large (or zero) a WRITE SAME com‐
145 mand may require considerably more time than 60 seconds to com‐
146 plete.
147 -U, --unmap
149 sets the UNMAP bit in the WRITE SAME(10, 16 and 32) cdb. See
150 UNMAP section below.
151 -v, --verbose
153 increase the degree of verbosity (debug messages).
154 -V, --version
156 output version string then exit.
157 -w, --wrprotect=WPR
159 sets the "Write protect" field in the WRITE SAME cdb to WPR. The
160 default value is zero. WPR should be a value between 0 and 7.
161 When WPR is 1 or greater, and the disk's protection type is 1 or
162 greater, then 8 extra bytes of protection information are
163 expected or generated (to place in the command's data-out buf‐
164 fer).
165 -x, --xferlen=LEN
167 where LEN is the data-out buffer length. Defaults to the length
168 of the IF file or, if that is not given, then the READ CAPAC‐
169 ITY(16 or 10) command is used to find the 'Logical block length
170 in bytes'. That figure may be increased by 8 bytes if the
171 DEVICE's protection type is 1 or greater and the WRPROTECT field
172 (see --wrprotect=WPR) is 1 or greater. If both this option and
173 the IF option are given and LEN exceeds the length of the IF
174 file then LEN is the data-out buffer length with zeros used as
175 pad bytes.
176
178 Logical block provisioning is a new term introduced in SBC-3 revision
179 25 for the ability to mark blocks as unused. For large storage arrays,
180 it is a way to provision less physical storage than the READ CAPACITY
181 command reports is available, potentially allocating more physical
182 storage when WRITE commands require it. For flash memory (e.g. SSD
183 drives) it is a way of potentially saving power (and perhaps access
184 time) when it is known large sections (or almost all) of the flash mem‐
185 ory is not in use. SSDs need wear levelling algorithms to have accept‐
186 able endurance and typically over provision to simplify those algo‐
187 rithms; hence they typically contain more physical flash storage than
188 their logical size would dictate.
189 Support for logical block provisioning is indicated by the LBPME bit
191 being set in the READ CAPACITY(16) command response (see the sg_readcap
192 utility). That implies at least one of the UNMAP or WRITE SAME(16)
193 commands is implemented. If the UNMAP command is implemented then the
194 "Maximum unmap LBA count" and "Maximum unmap block descriptor count"
195 fields in the Block Limits VPD page should both be greater than zero.
196 The READ CAPACITY(16) command response also contains a LBPRZ bit which
197 if set means that if unmapped blocks are read then zeros will be
198 returned for the data (and if protection information is active, 0xff
199 bytes are returned for that). In SBC-3 revision 27 the same LBPRZ bit
200 was added to the Logical Block Provisioning VPD page.
201 In SBC-3 revision 25 the LBPU and ANC_SUP bits where added to the Logi‐
203 cal Block Provisioning VPD page. When LBPU is set it indicates that the
204 device supports the UNMAP command (see the sg_unmap utility). When the
205 ANC_SUP bit is set it indicates the device supports anchored LBAs.
206 When the UNMAP bit is set in the cdb then the data-out buffer is also
208 sent. Additionally the data section of that data-out buffer should be
209 full of 0x0 bytes while the data protection block, 8 bytes at the end
210 if present, should be set to 0xff bytes. If these conditions are not
211 met and the LBPRZ bit is set then the UNMAP bit is ignored and the
212 data-out buffer is written to the DEVICE as if the UNMAP bit was zero.
213 In the absence of the --in=IF option, this utility will attempt build a
214 data-out buffer that meets the requirements for the UNMAP bit in the
215 cdb to be acted on by the DEVICE.
216 Logical blocks may also be unmapped by the SCSI UNMAP and FORMAT UNIT
218 commands (see the sg_unmap and sg_format utilities).
219 The unmap capability in SCSI is closely related to the ATA DATA SET
221 MANAGEMENT command with the "Trim" bit set. That ATA trim capability
222 does not interact well with SATA command queueing known as NCQ. T13
223 have introduced a new command called the SFQ DATA SET MANAGEMENT com‐
224 mand also with a the "Trim" bit to address that problem. The SCSI WRITE
225 SAME with the UNMAP bit set and the UNMAP commands do not have any
226 problems with SCSI queueing.
227
229 Various numeric arguments (e.g. LBA) may include multiplicative suf‐
230 fixes or be given in hexadecimal. See the "NUMERIC ARGUMENTS" section
231 in the sg3_utils(8) man page.
232 In Linux, prior to lk 3.17, the sg driver did not support cdb sizes
234 greater than 16 bytes. Hence a device node like /dev/sg1 which is asso‐
235 ciated with the sg driver would fail with this utility if the --32
236 option was given (or implied by other options). The bsg driver with
237 device nodes like /dev/bsg/6:0:0:1 does support cdb sizes greater than
238 16 bytes since its introduction in lk 2.6.28 .
239
241 The exit status of sg_write_same is 0 when it is successful. Otherwise
242 see the sg3_utils(8) man page.
243
245 One simple usage is to write blocks of zero from (and including) a
246 given LBA:
247 sg_write_same --lba=0x1234 --num=63 /dev/sdc
249 Since --xferlen=LEN has not been given, then this utility will call the
251 READ CAPACITY command on /dev/sdc to determine the number of bytes in a
252 logical block. Let us assume that is 512 bytes. Since --in=IF is not
253 given a block of zeros is assumed. So 63 blocks of zeros (each block
254 containing 512 bytes) will be written from (and including) LBA 0x1234 .
255 Note that only one block of zeros is passed to the SCSI WRITE SAME com‐
256 mand in the data-out buffer (as required by SBC-3).
257 A similar example follows but in this case the blocks are "unmapped"
259 ("trimmed" in ATA speak) rather than zeroed:
260 sg_write_same --unmap -L 0x1234 -n 63 /dev/sdc
262 Note that if the LBPRZ bit in the READ CAPACITY(16) response is set
264 (i.e. LPPRZ is an acronym for logical block provisioning read zeros)
265 then these two examples do the same thing, at least seen from the point
266 of view of subsequent reads.
267 This utility can also be used to write protection information (PI) on
269 disks formatted with a protection type greater than zero. PI is 8 bytes
270 of extra data appended to the user data of a logical block: the first
271 two bytes are a CRC (the "guard"), the next two bytes are the "applica‐
272 tion tag" and the last four bytes are the "reference tag". With protec‐
273 tion types 1 and 2 if the application tag is 0xffff then the guard
274 should not be checked (against the user data).
275 In this example we assume the logical block size (of the user data) is
277 512 bytes and the disk has been formatted with protection type 1. Since
278 we are going to modify LBA 2468 then we take a copy of it first:
279 dd if=/dev/sdb skip=2468 bs=512 of=2468.bin count=1
281 The following command line sets the user data to zeros and the PI to 8
283 0xFF bytes on LBA 2468:
284 sg_write_same --lba=2468 /dev/sdb
286 Reading back that block should be successful because the application
288 tag is 0xffff which suppresses the guard (CRC) check (which would oth‐
289 erwise be wrong):
290 dd if=/dev/sdb skip=2468 bs=512 of=/dev/null count=1
292 Now an attempt is made to create a binary file with zeros in the user
294 data, 0x0000 in the application tag and 0xff bytes in the other two PI
295 fields. It is awkward to create 0xff bytes in a file (in Unix) as the
296 "tr" command below shows:
297 dd if=/dev/zero bs=1 count=512 of=ud.bin
299 tr "\000" "\377" < /dev/zero | dd bs=1 of=ff_s.bin count=8
300 cat ud.bin ff_s.bin > lb.bin
301 dd if=/dev/zero bs=1 count=2 seek=514 conv=notrunc of=lb.bin
302 The resulting file can be viewed with 'hexdump -C lb.bin' and should
304 contain 520 bytes. Now that file can be written to LBA 2468 as follows:
305 sg_write_same --lba=2468 wrprotect=3 --in=lb.bin /dev/sdb
307 Note the --wrprotect=3 rather than being set to 1, since we want the
309 WRITE SAME command to succeed even though the PI data now indicates the
310 user data is corrupted. When an attempt is made to read the LBA, an
311 error should occur:
312 dd if=/dev/sdb skip=2468 bs=512 of=/dev/null count=1
314 dd errors are not very expressive, if dmesg is checked there should be
316 a line something like this: "[sdb] Add. Sense: Logical block guard
317 check failed". The block can be corrected by doing a "sg_write_same
318 --lba=1234 /dev/sdb" again or restoring the original contents of that
319 LBA:
320 dd if=2468.bin bs=512 seek=2468 of=/dev/sdb conv=notrunc count=1
322 Hopefully the dd command would never try to truncate the output file
324 when it is a block device.
325
327 Written by Douglas Gilbert.
328
330 Report bugs to <dgilbert at interlog dot com>.
331
333 Copyright © 2009-2019 Douglas Gilbert
334 This software is distributed under a FreeBSD license. There is NO war‐
335 ranty; not even for MERCHANTABILITY or FITNESS FOR A PARTICULAR PUR‐
336 POSE.
337
339 sg_format,sg_get_lba_status,sg_readcap,sg_vpd,sg_unmap(sg3_utils)
340sg3_utils-1.45 December 2019 SG_WRITE_SAME(8)