[lvm-devel] master - scan: expand and update label scan comments
David Teigland
teigland at sourceware.org
Tue May 21 17:21:23 UTC 2019
Gitweb: https://sourceware.org/git/?p=lvm2.git;a=commitdiff;h=dc1e12dcd423cf912489e1ed380fa6eb1a049f2e
Commit: dc1e12dcd423cf912489e1ed380fa6eb1a049f2e
Parent: 60bf9c9f331fb0282b1ceca57148aad54cb943a0
Author: David Teigland <teigland at redhat.com>
AuthorDate: Tue May 21 12:02:40 2019 -0500
Committer: David Teigland <teigland at redhat.com>
CommitterDate: Tue May 21 12:02:40 2019 -0500
scan: expand and update label scan comments
---
lib/label/label.c | 89 ++++++++++++++++++++++++++++++++++++----------------
1 files changed, 61 insertions(+), 28 deletions(-)
diff --git a/lib/label/label.c b/lib/label/label.c
index b999f87..43b05a2 100644
--- a/lib/label/label.c
+++ b/lib/label/label.c
@@ -847,8 +847,11 @@ static void _free_hints(struct dm_list *hints)
}
/*
- * Scan and cache lvm data from all devices on the system.
- * The cache should be empty/reset before calling this.
+ * Scan devices on the system to discover which are LVM devices.
+ * Info about the LVM devices (PVs) is saved in lvmcache in a
+ * basic/summary form (info/vginfo structs). The vg_read phase
+ * uses this summary info to know which PVs to look at for
+ * processing a given VG.
*/
int label_scan(struct cmd_context *cmd)
@@ -860,7 +863,8 @@ int label_scan(struct cmd_context *cmd)
struct device_list *devl, *devl2;
struct device *dev;
uint64_t max_metadata_size_bytes;
- int newhints = 0;
+ int using_hints;
+ int create_hints = 0; /* NEWHINTS_NONE */
log_debug_devs("Finding devices to scan");
@@ -869,20 +873,37 @@ int label_scan(struct cmd_context *cmd)
dm_list_init(&hints_list);
/*
- * Iterate through all the devices in dev-cache (block devs that appear
- * under /dev that could possibly hold a PV and are not excluded by
- * filters). Read each to see if it's an lvm device, and if so
- * populate lvmcache with some basic info about the device and the VG
- * on it. This info will be used by the vg_read() phase of the
- * command.
+ * dev_cache_scan() creates a list of devices on the system
+ * (saved in in dev-cache) which we can iterate through to
+ * search for LVM devs. The dev cache list either comes from
+ * looking at dev nodes under /dev, or from udev.
*/
dev_cache_scan();
+ /*
+ * Set up the iterator that is needed to step through each device in
+ * dev cache.
+ */
if (!(iter = dev_iter_create(cmd->filter, 0))) {
log_error("Scanning failed to get devices.");
return 0;
}
+ log_debug_devs("Filtering devices to scan");
+
+ /*
+ * Iterate through all devices in dev cache and apply filters
+ * to exclude devs that we do not need to scan. Those devs
+ * that pass the filters are returned by the iterator and
+ * saved in a list of devs that we will proceed to scan to
+ * check if they are LVM devs. IOW this loop is the
+ * application of filters (those that do not require reading
+ * the devs) to the list of all devices. It does that because
+ * the 'cmd->filter' is used above when setting up the iterator.
+ * Unfortunately, it's not obvious that this is what's happening
+ * here. filters that require reading the device are not applied
+ * here, but in process_block(), see DEV_FILTER_AFTER_SCAN.
+ */
while ((dev = dev_iter_get(cmd, iter))) {
if (!(devl = zalloc(sizeof(*devl))))
continue;
@@ -901,13 +922,12 @@ int label_scan(struct cmd_context *cmd)
/*
* When md devices exist that use the old superblock at the
* end of the device, then in order to detect and filter out
- * the component devices of those md devs, we need to enable
- * the full md filter which scans both the start and the end
- * of every device. This doubles the amount of scanning i/o,
- * which we want to avoid. FIXME: it may not be worth the
- * cost of double i/o just to avoid displaying md component
- * devs in 'pvs', which is a pretty harmless effect from a
- * pretty uncommon situation.
+ * the component devices of those md devs, we enable the full
+ * md filter which scans both the start and the end of every
+ * device. This doubles the amount of scanning i/o, which we
+ * want to avoid. FIXME: this forces start+end scanning of
+ * every device, but it would be more efficient to limit the
+ * end scan only to PVs.
*/
if (dev_is_md_with_end_superblock(cmd->dev_types, dev))
cmd->use_full_md_check = 1;
@@ -920,7 +940,10 @@ int label_scan(struct cmd_context *cmd)
}
/*
- * In some common cases we can avoid scanning all devices.
+ * In some common cases we can avoid scanning all devices
+ * by using hints which tell us which devices are PVs, which
+ * are the only devices we actually need to scan. Without
+ * hints we need to scan all devs to find which are PVs.
*
* TODO: if the command is using hints and a single vgname
* arg, we can also take the vg lock here, prior to scanning.
@@ -930,10 +953,12 @@ int label_scan(struct cmd_context *cmd)
* able to avoid rescan in vg_read, but locking early would
* apply to more cases.)
*/
- if (!get_hints(cmd, &hints_list, &newhints, &all_devs, &scan_devs)) {
+ if (!get_hints(cmd, &hints_list, &create_hints, &all_devs, &scan_devs)) {
dm_list_splice(&scan_devs, &all_devs);
dm_list_init(&hints_list);
- }
+ using_hints = 0;
+ } else
+ using_hints = 1;
log_debug("Will scan %d devices skip %d", dm_list_size(&scan_devs), dm_list_size(&all_devs));
@@ -979,17 +1004,18 @@ int label_scan(struct cmd_context *cmd)
dm_list_init(&cmd->hints);
- if (!dm_list_empty(&hints_list)) {
+ /*
+ * If we're using hints to limit which devs we scanned, verify
+ * that those hints were valid, and if not we need to scan the
+ * rest of the devs.
+ */
+ if (using_hints) {
if (!validate_hints(cmd, &hints_list)) {
- /*
- * We scanned a subset of all devices based on hints.
- * With the results from the scan we may decide that
- * the hints are not valid, so scan all others.
- */
log_debug("Will scan %d remaining devices", dm_list_size(&all_devs));
_scan_list(cmd, cmd->filter, &all_devs, NULL);
_free_hints(&hints_list);
- newhints = 0;
+ using_hints = 0;
+ create_hints = 0;
} else {
/* The hints may be used by another device iteration. */
dm_list_splice(&cmd->hints, &hints_list);
@@ -1006,8 +1032,15 @@ int label_scan(struct cmd_context *cmd)
free(devl);
}
- if (newhints)
- write_hint_file(cmd, newhints);
+ /*
+ * If hints were not available/usable, then we scanned all devs,
+ * and we now know which are PVs. Save this list of PVs we've
+ * identified as hints for the next command to use.
+ * (create_hints variable has NEWHINTS_X value which indicates
+ * the reason for creating the new hints.)
+ */
+ if (create_hints)
+ write_hint_file(cmd, create_hints);
return 1;
}
More information about the lvm-devel
mailing list