diff --git a/src/libostree/ostree-sysroot-deploy.c b/src/libostree/ostree-sysroot-deploy.c
index c7148208bc..5c941d5170 100644
--- a/src/libostree/ostree-sysroot-deploy.c
+++ b/src/libostree/ostree-sysroot-deploy.c
@@ -59,6 +59,12 @@
   SD_ID128_MAKE (e8, 64, 6c, d6, 3d, ff, 46, 25, b7, 79, 09, a8, e7, a4, 09, 94)
 #endif
 
+/* How much additional space we require available on top of what we accounted
+ * during the early prune fallocate space check. This accounts for anything not
+ * captured directly by `get_kernel_layout_size()` like writing new BLS entries.
+ */
+#define EARLY_PRUNE_SAFETY_MARGIN_SIZE (1 << 20) /* 1 MB */
+
 /*
  * Like symlinkat() but overwrites (atomically) an existing
  * symlink.
@@ -2450,7 +2456,8 @@ write_deployments_finish (OstreeSysroot *self, GCancellable *cancellable, GError
 }
 
 static gboolean
-add_file_size_if_nonnull (int dfd, const char *path, guint64 *inout_size, GError **error)
+add_file_size_if_nonnull (int dfd, const char *path, guint64 blocksize, guint64 *inout_size,
+                          GError **error)
 {
   if (path == NULL)
     return TRUE;
@@ -2460,14 +2467,21 @@ add_file_size_if_nonnull (int dfd, const char *path, guint64 *inout_size, GError
     return FALSE;
 
   *inout_size += stbuf.st_size;
+  if (blocksize > 0)
+    {
+      off_t rem = stbuf.st_size % blocksize;
+      if (rem > 0)
+        *inout_size += blocksize - rem;
+    }
+
   return TRUE;
 }
 
 /* calculates the total size of the bootcsum dir in /boot after we would copy
  * it. This reflects the logic in  install_deployment_kernel(). */
 static gboolean
-get_kernel_layout_size (OstreeSysroot *self, OstreeDeployment *deployment, guint64 *out_size,
-                        GCancellable *cancellable, GError **error)
+get_kernel_layout_size (OstreeSysroot *self, OstreeDeployment *deployment, guint64 blocksize,
+                        guint64 *out_size, GCancellable *cancellable, GError **error)
 {
   g_autofree char *deployment_dirpath = ostree_sysroot_get_deployment_dirpath (self, deployment);
   glnx_autofd int deployment_dfd = -1;
@@ -2479,11 +2493,11 @@ get_kernel_layout_size (OstreeSysroot *self, OstreeDeployment *deployment, guint
     return FALSE;
 
   guint64 bootdir_size = 0;
-  if (!add_file_size_if_nonnull (kernel_layout->boot_dfd, kernel_layout->kernel_srcpath,
+  if (!add_file_size_if_nonnull (kernel_layout->boot_dfd, kernel_layout->kernel_srcpath, blocksize,
                                  &bootdir_size, error))
     return FALSE;
   if (!add_file_size_if_nonnull (kernel_layout->boot_dfd, kernel_layout->initramfs_srcpath,
-                                 &bootdir_size, error))
+                                 blocksize, &bootdir_size, error))
     return FALSE;
   if (kernel_layout->devicetree_srcpath)
     {
@@ -2491,22 +2505,22 @@ get_kernel_layout_size (OstreeSysroot *self, OstreeDeployment *deployment, guint
       if (kernel_layout->devicetree_namever)
         {
           if (!add_file_size_if_nonnull (kernel_layout->boot_dfd, kernel_layout->devicetree_srcpath,
-                                         &bootdir_size, error))
+                                         blocksize, &bootdir_size, error))
             return FALSE;
         }
       else
         {
           guint64 dirsize = 0;
           if (!ot_get_dir_size (kernel_layout->boot_dfd, kernel_layout->devicetree_srcpath,
-                                &dirsize, cancellable, error))
+                                blocksize, &dirsize, cancellable, error))
             return FALSE;
           bootdir_size += dirsize;
         }
     }
   if (!add_file_size_if_nonnull (kernel_layout->boot_dfd, kernel_layout->kernel_hmac_srcpath,
-                                 &bootdir_size, error))
+                                 blocksize, &bootdir_size, error))
     return FALSE;
-  if (!add_file_size_if_nonnull (kernel_layout->boot_dfd, kernel_layout->aboot_srcpath,
+  if (!add_file_size_if_nonnull (kernel_layout->boot_dfd, kernel_layout->aboot_srcpath, blocksize,
                                  &bootdir_size, error))
     return FALSE;
 
@@ -2533,6 +2547,9 @@ dfd_fallocate_check (int dfd, off_t len, gboolean *out_passed, GError **error)
   if (!glnx_open_tmpfile_linkable_at (dfd, ".", O_WRONLY | O_CLOEXEC, &tmpf, error))
     return FALSE;
 
+  /* add the safety margin */
+  len += EARLY_PRUNE_SAFETY_MARGIN_SIZE;
+
   *out_passed = TRUE;
   /* There's glnx_try_fallocate, but not with the same error semantics. */
   if (TEMP_FAILURE_RETRY (fallocate (tmpf.fd, 0, 0, len)) < 0)
@@ -2583,6 +2600,11 @@ auto_early_prune_old_deployments (OstreeSysroot *self, GPtrArray *new_deployment
   g_autoptr (GHashTable) new_bootcsums
       = g_hash_table_new_full (g_str_hash, g_str_equal, g_free, NULL);
 
+  /* get bootfs block size */
+  struct statvfs stvfsbuf;
+  if (TEMP_FAILURE_RETRY (fstatvfs (self->boot_fd, &stvfsbuf)) < 0)
+    return glnx_throw_errno_prefix (error, "fstatvfs(boot)");
+
   g_auto (GStrv) bootdirs = NULL;
   if (!_ostree_sysroot_list_all_boot_directories (self, &bootdirs, cancellable, error))
     return glnx_prefix_error (error, "listing bootcsum directories in bootfs");
@@ -2597,7 +2619,8 @@ auto_early_prune_old_deployments (OstreeSysroot *self, GPtrArray *new_deployment
 
       guint64 bootdir_size;
       g_autofree char *ostree_bootdir = g_build_filename ("ostree", bootdir, NULL);
-      if (!ot_get_dir_size (self->boot_fd, ostree_bootdir, &bootdir_size, cancellable, error))
+      if (!ot_get_dir_size (self->boot_fd, ostree_bootdir, stvfsbuf.f_bsize, &bootdir_size,
+                            cancellable, error))
         return FALSE;
 
       /* for our purposes of sizing bootcsums, it's highly unlikely we need a
@@ -2609,10 +2632,7 @@ auto_early_prune_old_deployments (OstreeSysroot *self, GPtrArray *new_deployment
            * that users report it and we tweak this code to handle this.
            *
            * An alternative is working with the block size instead, which would
-           * be easier to handle. But ideally, `ot_get_dir_size` would be block
-           * size aware too for better accuracy, which is awkward since the
-           * function itself is generic over directories and doesn't consider
-           * e.g. mount points from different filesystems. */
+           * be easier to handle. */
           g_printerr ("bootcsum %s size exceeds %u; disabling auto-prune optimization\n", bootdir,
                       G_MAXUINT);
           return TRUE;
@@ -2640,7 +2660,8 @@ auto_early_prune_old_deployments (OstreeSysroot *self, GPtrArray *new_deployment
         }
 
       guint64 bootdir_size = 0;
-      if (!get_kernel_layout_size (self, deployment, &bootdir_size, cancellable, error))
+      if (!get_kernel_layout_size (self, deployment, stvfsbuf.f_bsize, &bootdir_size, cancellable,
+                                   error))
         return FALSE;
 
       /* see similar logic in previous loop */
diff --git a/src/libotutil/ot-fs-utils.c b/src/libotutil/ot-fs-utils.c
index efbb2f207e..1e961a986c 100644
--- a/src/libotutil/ot-fs-utils.c
+++ b/src/libotutil/ot-fs-utils.c
@@ -227,11 +227,12 @@ ot_parse_file_by_line (const char *path, gboolean (*cb) (const char *, void *, G
   return TRUE;
 }
 
-/* Calculate the size of the files contained in a directory. Symlinks are not
- * followed. */
+/* Calculate the size of the files contained in a directory. Symlinks are
+ * not followed. If `blocksize` is nonzero, all sizes are rounded to its next
+ * multiple. */
 gboolean
-ot_get_dir_size (int dfd, const char *path, guint64 *out_size, GCancellable *cancellable,
-                 GError **error)
+ot_get_dir_size (int dfd, const char *path, guint64 blocksize, guint64 *out_size,
+                 GCancellable *cancellable, GError **error)
 {
   g_auto (GLnxDirFdIterator) dfd_iter = {
     0,
@@ -256,11 +257,18 @@ ot_get_dir_size (int dfd, const char *path, guint64 *out_size, GCancellable *can
             return FALSE;
 
           *out_size += stbuf.st_size;
+          if (blocksize > 0)
+            {
+              off_t rem = stbuf.st_size % blocksize;
+              if (rem > 0)
+                *out_size += blocksize - rem;
+            }
         }
       else if (dent->d_type == DT_DIR)
         {
           guint64 subdir_size;
-          if (!ot_get_dir_size (dfd_iter.fd, dent->d_name, &subdir_size, cancellable, error))
+          if (!ot_get_dir_size (dfd_iter.fd, dent->d_name, blocksize, &subdir_size, cancellable,
+                                error))
             return FALSE;
 
           *out_size += subdir_size;
diff --git a/src/libotutil/ot-fs-utils.h b/src/libotutil/ot-fs-utils.h
index b64adce247..7df79ba2a6 100644
--- a/src/libotutil/ot-fs-utils.h
+++ b/src/libotutil/ot-fs-utils.h
@@ -75,7 +75,7 @@ GBytes *ot_fd_readall_or_mmap (int fd, goffset offset, GError **error);
 gboolean ot_parse_file_by_line (const char *path, gboolean (*cb) (const char *, void *, GError **),
                                 void *cbdata, GCancellable *cancellable, GError **error);
 
-gboolean ot_get_dir_size (int dfd, const char *path, guint64 *out_size, GCancellable *cancellable,
-                          GError **error);
+gboolean ot_get_dir_size (int dfd, const char *path, guint64 blocksize, guint64 *out_size,
+                          GCancellable *cancellable, GError **error);
 
 G_END_DECLS
diff --git a/tests/kolainst/destructive/auto-prune.sh b/tests/kolainst/destructive/auto-prune.sh
index 1ec1534e52..5081275e07 100755
--- a/tests/kolainst/destructive/auto-prune.sh
+++ b/tests/kolainst/destructive/auto-prune.sh
@@ -106,6 +106,7 @@ rpm-ostree rollback
 # to not actually fit (because some filesystems like ext4 include reserved
 # overhead in their f_bfree count for some reason) will still trigger the auto-
 # prune logic.
+# https://github.com/ostreedev/ostree/pull/2866
 
 unconsume_bootfs_space
 
@@ -114,10 +115,10 @@ unconsume_bootfs_space
 unshare -m bash -c \
   "mount -o rw,remount /boot && \
    cp /usr/lib/modules/`uname -r`/{vmlinuz,initramfs.img} /boot"
-free_blocks=$(stat --file-system /boot -c '%f')
+free_blocks_kernel_and_initrd=$(stat --file-system /boot -c '%f')
 unshare -m bash -c \
   "mount -o rw,remount /boot && rm /boot/{vmlinuz,initramfs.img}"
-consume_bootfs_space "$((free_blocks))"
+consume_bootfs_space "$((free_blocks_kernel_and_initrd))"
 
 rpm-ostree rebase :modkernel1
 # Disable auto-pruning to verify we reproduce the bug
@@ -133,4 +134,32 @@ ostree admin finalize-staged |& tee out.txt
 assert_file_has_content out.txt "updating bootloader in two steps"
 rm out.txt
 
+# Below, we test that the size estimator is blocksize aware. This catches the
+# case where the dtb contains many small files such that there's a lot of wasted
+# block space we need to account for.
+# https://github.com/coreos/fedora-coreos-tracker/issues/1637
+
+unconsume_bootfs_space
+
+mkdir -p rootfs/usr/lib/modules/`uname -r`/dtb
+(set +x; for i in {1..10000}; do echo -n x > rootfs/usr/lib/modules/`uname -r`/dtb/$i; done)
+ostree commit --base modkernel1 -P --tree=dir=rootfs -b modkernel3
+
+# a naive estimator would think all those files just take 10000 bytes
+consume_bootfs_space "$((free_blocks_kernel_and_initrd - 10000))"
+
+rpm-ostree rebase :modkernel3
+# Disable auto-pruning to verify we reproduce the bug
+if OSTREE_SYSROOT_OPTS=no-early-prune ostree admin finalize-staged |& tee out.txt; then
+    assert_not_reached "successfully wrote kernel without auto-pruning"
+fi
+assert_file_has_content out.txt "No space left on device"
+rm out.txt
+
+# now, try again but with (now default) auto-pruning enabled
+rpm-ostree rebase :modkernel3
+ostree admin finalize-staged |& tee out.txt
+assert_file_has_content out.txt "updating bootloader in two steps"
+rm out.txt
+
 echo "ok bootfs auto-prune"