11:01 p.m.
Servers such as qemu can already tell us their preferred block size,
and work is underway to let nbdkit do likewise. As this information
can prove useful to clients, we need a new API to expose it. The API
is designed to support future enum values if the NBD protocol is ever
extended further to advertise larger limits for trim, zero, and cache
operations.
---
generator/API.ml | 70 +++++++++++++++++++++++++++---
generator/states-newstyle-opt-go.c | 1 +
lib/flags.c | 18 +++++++-
3 files changed, 83 insertions(+), 6 deletions(-)
diff --git a/generator/API.ml b/generator/API.ml
index 0b6c748..985d715 100644
--- a/generator/API.ml
+++ b/generator/API.ml
@@ -138,7 +138,15 @@ let tls_enum = {
"REQUIRE", 2;
]
}
-let all_enums = [ tls_enum ]
+let block_size_enum = {
+ enum_prefix = "SIZE";
+ enums = [
+ "MINIMUM", 0;
+ "PREFERRED", 1;
+ "MAXIMUM", 2;
+ ]
+}
+let all_enums = [ tls_enum; block_size_enum ]
(* Flags. See also Constants below. *)
let cmd_flags = {
@@ -1227,7 +1235,8 @@ Most modern NBD servers use C<\"newstyle-fixed\">.
^ non_blocking_test_call_description;
see_also = [Link "get_handshake_flags";
Link "get_structured_replies_negotiated";
- Link "get_tls_negotiated"];
+ Link "get_tls_negotiated";
+ Link "get_block_size"];
};
"get_size", {
@@ -1242,6 +1251,55 @@ Returns the size in bytes of the NBD export."
example = Some "examples/get-size.c";
};
+ "get_block_size", {
+ default_call with
+ args = [Enum ("size_type", block_size_enum)]; ret = RInt64;
+ permitted_states = [ Connected; Closed ];
+ shortdesc = "return a specific server block size constraint";
+ longdesc = "\
+Returns a specific size constraint advertised by the server, if any. If
+the return is zero, the server did not advertise a constraint. C<size_type>
+must be one of the following constraints:
+
+=over 4
+
+=item C<LIBNBD_SIZE_MINIMUM> = 0
+
+If non-zero, this will be a power of 2 between 1 and 64k; any client
+request that is not aligned in length or offset to this size is likely
+to fail with C<EINVAL>. The image size will generally also be a
+multiple of this value (if not, the final few bytes are inaccessible
+while obeying alignment constraints). If zero, it is safest to
+assume a minimum block size of 512, although many servers support
+a minimum block size of 1.
+
+=item C<LIBNBD_SIZE_PREFERRED> = 1
+
+If non-zero, this is a power of 2 representing the preferred size for
+efficient I/O. Smaller requests may incur overhead such as
+read-modify-write cycles that will not be present when using I/O that
+is a multiple of this value. This value may be larger than the size
+of the export. If zero, using 4k as a preferred block size tends to
+give decent performance.
+
+=item C<LIBNBD_SIZE_MAXIMUM> = 2
+
+If non-zero, this represents the maximum length that the server is
+willing to handle during C<nbd_pread> or C<nbd_pwrite>. Other functions
+like C<nbd_zero> may still be able to use larger sizes. Note that
+this function returns what the server advertised, but libnbd itself
+imposes a maximum of 64M. If zero, some NBD servers will abruptly
+disconnect if a transaction involves more than 32M.
+
+=back
+
+Future NBD extensions may result in additional C<size_type> values.
+"
+^ non_blocking_test_call_description;
+ see_also = [Link "get_protocol";
+ Link "get_size"]
+ };
+
"pread", {
default_call with
args = [ BytesOut ("buf", "count"); UInt64 "offset" ];
@@ -1260,7 +1318,8 @@ C<LIBNBD_CMD_FLAG_DF>.
The C<flags> parameter must be C<0> for now (it exists for future NBD
protocol extensions).";
- see_also = [Link "aio_pread"; Link "pread_structured"];
+ see_also = [Link "aio_pread"; Link "pread_structured";
+ Link "get_block_size"];
example = Some "examples/fetch-first-sector.c";
};
@@ -1338,7 +1397,7 @@ more than one fragment (if that is supported - some servers cannot
do
this, see L<nbd_can_df(3)>). Libnbd does not validate that the server
actually obeys the flag.";
see_also = [Link "can_df"; Link "pread";
- Link "aio_pread_structured"];
+ Link "aio_pread_structured"; Link "get_block_size"];
};
"pwrite", {
@@ -1361,7 +1420,7 @@ return until the data has been committed to permanent storage
(if that is supported - some servers cannot do this, see
L<nbd_can_fua(3)>).";
see_also = [Link "can_fua"; Link "is_read_only";
- Link "aio_pwrite"];
+ Link "aio_pwrite"; Link "get_block_size"];
example = Some "examples/reads-and-writes.c";
};
@@ -2277,6 +2336,7 @@ let first_version = [
"get_list_exports", (1, 4);
"get_nr_list_exports", (1, 4);
"get_list_export_name", (1, 4);
+ "get_block_size", (1, 4);
(* These calls are proposed for a future version of libnbd, but
* have not been added to any released version so far.
diff --git a/generator/states-newstyle-opt-go.c b/generator/states-newstyle-opt-go.c
index 39406fa..501144d 100644
--- a/generator/states-newstyle-opt-go.c
+++ b/generator/states-newstyle-opt-go.c
@@ -151,6 +151,7 @@ STATE_MACHINE {
h->block_minimum = be32toh (h->sbuf.or.payload.block_size.minimum);
h->block_preferred = be32toh (h->sbuf.or.payload.block_size.preferred);
h->block_maximum = be32toh (h->sbuf.or.payload.block_size.maximum);
+ /* XXX Sanity check that server values are sane... */
break;
default:
/* XXX Handle other info types, like NBD_INFO_DESCRIPTION */
diff --git a/lib/flags.c b/lib/flags.c
index d55d10a..208097b 100644
--- a/lib/flags.c
+++ b/lib/flags.c
@@ -1,5 +1,5 @@
/* NBD client library in userspace
- * Copyright (C) 2013-2019 Red Hat Inc.
+ * Copyright (C) 2013-2020 Red Hat Inc.
*
* This library is free software; you can redistribute it and/or
* modify it under the terms of the GNU Lesser General Public
@@ -157,3 +157,19 @@ nbd_unlocked_get_size (struct nbd_handle *h)
return h->exportsize;
}
+
+int64_t nbd_unlocked_get_block_size (struct nbd_handle *h, int type)
+{
+ /* XXX Should we sanitize these, or provide sane defaults when server
+ * did not?
+ */
+ switch (type) {
+ case LIBNBD_SIZE_MINIMUM:
+ return h->block_minimum;
+ case LIBNBD_SIZE_PREFERRED:
+ return h->block_preferred;
+ case LIBNBD_SIZE_MAXIMUM:
+ return h->block_maximum;
+ }
+ return 0;
+}
--
2.27.0